No video signal after about 30-60s

Hi guys, hope someone can help with this problem:

My PC froze today while playing a video, lost video signal and I had to force a reboot. Now I find that there is no video signal, although the monitor and cable work fine when attached to my laptop.

If I leave the computer off for about 15 minutes, then reboot, the monitor does display the normal boot screen, then loses signal again about 30-60s into the boot sequence (by losing signal, I mean that it says 'no video signal' and then goes black).

If I go into setup in the BIOS, I can see that the PSU voltages seem ok but it still loses signal in a similar space of time. It is a race against time to get any info from the BIOS before I lose it!

So, I'm wondering if it is a cooling problem ie. it is overheating and needs time to cool between boots. There are two fans on the motherboard (one on the board and one adjacent on the case). These are both going round, noisily. Quite dusty, so I have vacuumed them. The video card has a fan but I don't see it going round.

I don't really know anything about fans and overheating (if indeed this is the problem) so would really be grateful for any help. Wondering if just buying a new video card would be in order, in which case, how can I choose an inexpensive basic card for this motherboard?

Thanks for any help - if this is in the wrong forum feel free to move it.

System:
CPU AMD Athlon 64 3500+
Geoforce FX-5700LE 256MB
1024MB Memory
6 year old PC (2005)
 
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