Internet not found/won't connect at system bootup

I've been using Win7 x64 for several months now, without significant problems when it comes it internet/networking. But lately, when I boot my PC, more often than not something goes wrong with the network/internet connectivity. When I boot, often Windows doesn't find the internet connection, or, takes a very long time to do so. In order to fix it I have to manually disable & re-enable the onboard Realtek NIC thru device manager, or power cycle the cable modem, and/or reboot the system again, to get connectivity. I can't tell if the problem is the Ambit cable modem or the NIC. My signal quality and strength are just fine.. S/N is 37 dB, and downstream power is -2 dB.

Any ideas why my PC won't connect to internet at boot? How can I diagnose & fix this?

Edit: also my internet speed seems very slow and erratic...sometimes it's testing at 7 mbps down, other times less than 1. for no apparent reason.

Thanks for the suggestions. But I can't really see how updating drivers would change the situation, as it was working fine for 4 months on the old driver. And I've heard many horror stories about onboard Realtek NICs getting bricked by driver updates that don't work.

I don't really know how to reset the NIC to default settings, as I haven't touched those from how they originally were. I suppose I could delete the thing in device manager and let Windows reinstall it.

Overall, I suspect my cable modem; when I had an old dying cable modem it had symptoms like this... intermittent loss of signal and terribly inconsistent download speeds.