weird video problem on boot-up

I have a very unusual problem: Sometimes, when booting, the monitor only shows a blinking cursor in the top-left corner. No POST, nothing else. The weird thing is that the system appears to be booting quite normally, as sometimes, if I wait long enough (1-2 minutes tops) I can hear the startup sound. Meanwhile, the cursor is steadily blinking, uncaring of the frustration it is causing.

My system is the following:

Win 7 PRO 32bit
1gb ddr (2x 512mb, single channel)
ASrock K8NF3-VSTA soket 754 (AGP8x)
AMD 64 3000+ (2 GHz)
ATI Radeon HD 3450 (256mb, AGP8x)
350W power source
1 IDE DVDRW (master)
1 SATA HDD (7200rpm, ~120 usable GB)
1 PCI wifi 802.11n card
PS/2 keyboard
USB wireless mouse

I use on-board sound card, pc connected to monitor via VGA.

I admit that my system is old, but it can still handle windows 7 and it feels more responsive than XP.

Recently, the only change to the specs was the upgraded the video card. I previously had the terrible FX5200, which could barely handle non-transparent aero. I upgraded to the radeon hd 3450 (about 1-2 years old, bought it at a fair price) and everything started smoothly. The PC felt like new, everything was faster and smoother, no more jerkiness, and I could finally use transparency. and as a bonus, 1080p HD works like a charm and smooth like butter on this aging computer (6 years old at least).

The ATI 3450 card needed aditional power (the 4-pin connector) and the specs say minimum requirement is 350W which is what my power source also says on the label so this is covered I think (or rather hope).

I got 3.6 rating after this upgrade:
CPU-3.9
RAM-4.3
Graphics-3.6
Gaming-5.4
HDD-5.3

which is rather well for an aging PC I think.

The problem appeared shortly after I installed the new GPU. A few days later I mean. Randomly, on boot-up, the PC doesn't show POST, only a blinking cursor in the top-left corner. The HDD, DVD, everything sounds like normal boot. I even manage to hear the start-up sound of windows 7. I didn't actually test it, but I think that every time windows boots succesfully, only I can't get the image as all I see is the constantly blinking cursor.

To be absolutely clear on the issue, sometimes, from the moment I push the power button and the system is booting, all I see on screen is the blinking cursor. Nothing else. No POST info, no nothing. it just stays like that, blinking.

If I power off, then back on, it usually gets fixed and I get the usual POST and windows boot-up animation. But sometimes, I reset the thing 5 times in a row and I still got the blinking cursor. It's just random. And it's hard to tell if it gets fixed because it can work ok 2-3 days in a row then suddenly start blinking at me again.

What I can tell is that this didn't happen with the older card, the FX5200. So something happened when I installed the new GPU.

Could it be faulty GPU? Could the power source not be powerful enough for this GPU? could it be some kind of incompatibility between the motherboard and the GPU or between the GPU and something else? Can I do anything to find out what the problem is?

I don't want tu buy anything more for this pc. If it costs me too much to fix this, I would rather buy parts for a new PC (mb, cpu, ram, the usual.)

Any help will be greatly appreciated.

So you don't say if you have tried new drivers for your newer graphics card. It sort of sounds like a power problem. Did you try a differnt 4 pin connector.