Ethernet adapter problem? Slow browsing and downloading/ buffering vid

PC developed slow browsing/downloading and buffering of video. Speed goes wildly up and down every minute, indicating a fault. All other devices (laptop, tablets, PS4 and PS3) work as normal with fast speeds even wirelessly. I tried the following:

1) Ran anti-virus/malware scan with avast and then malware bytes and in safe mode also.

2) Tested with spares (ethernet phone cable/micro filter/RJ11 from router to PC).
Connected the router to the micro filter/BT phone master socket and even tried direct without the micro filter into the test socket.

3) Tried 2 other browsers and with antivirus uninstalled.

4) Deleted the Realtek PCIe GBE Family Controller driver and updated to the latest from the MSI website.

5) In Device Manager right clicked on ethernet/network adapter - Properties - Advanced tab; for the 'Speed & duplex' property tried changing to different values, beginning with 100 Mbps full duplex to 10 Mbps full duplex. IPv6 was enabled so I've disabled it and also in registry settings.

6) Bought a new SSD and did a fresh install of Windows 7



I custom built my PC and for the first three years it handled fast (for my area) broadband internet (12 MBPs) and 1080p video smoothly without any problems. Now all my other devices (laptop, tablets, PS4 and PS3) work as normal with fast speeds even wirelessly, so it seems my PC have developed NIC card (network interface controller)/ethernet adapter problems.

We mainly use the PC for browsing and BBC Iplayer. Then about a month ago video started buffering on sites extremely slowly (coincidentally after heavy rain). It would play for between 1-2 minutes for fine, and then pause to buffer for a minute or even longer, and it would do this continually, with the quality of the video changing to low and then changing back to HD. For some strange reason it didn't affect video sharing sites such as youtube/vimeo/dailymotion etc. We checked netflix and Prime video and the quality would change from HD to very fuzzy low resolution in a similar fashion, although the playback was smooth, it still had problems where as on PS4 and PS3 it is still flawless 1080p. We've since had to start experimenting on multiple unofficial websites to test the problem, and they all buffer the same as Iplayer. I've tried a different browser and that made no difference either.

I thought I would try testing if my spare cables would make a difference, but they didn't. Also when I unplugged and re-plugged the ethernet phone cable (connecting the router to the micro filter/BT phone
master socket and even tried without the micro filter directly plugged into the test socket) for some reason the PC wouldn't connect to the internet (the internet symbol showed a yellow exclamation mark instead of the PC symbol). I unplugged and re-plugged it again and at one point it even notified me that the ethernet cable wasn't plugged in! It took a few minutes after which it connected (without me doing anything).

But the buffering problem remained. Since then the problem has got a bit better occasionally (temporarily), and sometimes a lot worse, but it hasn't gone away.

I am with Origin Broadband and my average speeds are 9.3 Mbps, and in general since this problem has started the strange thing is that it hasn't affected the speeds recorded on speedtest.net, I do now get drastic drops to around 3 Mbps or even 1 Mbps occasionally but they only last a few moments-I re-do the tests and they are back to normal speeds, so I'm not sure if they're an error in the test.

The strangest thing is that it happens on PC, but not on other devices connected such as tablets and the laptop which all run at full speed as normal. I've never experienced this before! It even stops me downloading anything, if I try to download a 200MB file it will take hours and keep failing, stating network error. I think it's an ethernet issue (Realtek PCIe GBE Family Controller) so I deleted the driver and updated to the latest from the MSI website but it made no difference. I also bought a new SSD and did a fresh install of Windows 7 on that but it hasn't changed anything.

I am now thinking the only solution is to buy another motherboard to get a new ethernet adapter, but am worried if even that doesn't fix the problem I would have wasted money. I contacted Scan and was told the motherboard was out of warranty, but buying a 3rd party external USB ethernet adapter wouldn't actually 'replace' the onboard one, so it would only provide a solution if the actual physical port to the onboard adapter is damaged (and not the adapter itself).

I can't see any damage on the actual physical port/connector where the ethernet cable plugs in. I'd love to be able to buy some cheap ethernet adapter that would fix the problem but am guessing that won't help so instead of possibly wasting money on something I'm thinking I should just look for a cheap motherboard compatible with my current build, unless someone can help diagnose/fix the problem with an alternative solution. Any ideas ?

My Build:
CPU: Intel - Xeon E3-1231 V3 3.4GHz Quad-Core Processor (£230.77 @ PC World Business)
Motherboard: MSI - Z97 PC MATE ATX LGA1150 Motherboard
On-board network interface controller: Realtek PCIe GBE Family Controller
Memory: Kingston - HyperX Fury Black 8GB (1 x 8GB) DDR3-1866 Memory (£58.36 @ More Computers)
Storage: Seagate - Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£81.24 @ More Computers)
Video Card: MSI - Radeon R9 285 2GB Video Card
Case: Corsair - SPEC-03 Red ATX Mid Tower Case (£44.99 @ Novatech)
Power Supply: XFX - TS 650W 80+ Gold Certified ATX Power Supply (£101.38 @ Amazon UK)
Monitor: BenQ - GL2250HM 21.5" 1920x1080 60Hz Monitor (£89.99 @ Amazon UK)
Keyboard: Cooler Master - CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse (£39.99 @ Amazon UK)
OS: Windows 7 Ultimate
Antivirus: Avast free