Possible graphics card issue?

Firstly, if this is in the wrong section i apologise!

When i first got my desktop (around 3-4years ago) i could run games without any performance issues. however in the last year or so i've been unable to play games without getting some horrendous lag spikes. the types of games vary from simple flash games and facebook apps to online rpg games and disks (such as age of empires).

After a couple minutes of playing i will have constant lag spikes (every minute or so, lasting between 20-40 seconds). i have checked the minimum specs required to play and my computer meets these requirements (will post).

when it first happened i had someone look at it and he said my computer was overheating (namely, the soldering on the heat sync was actually melting) and dust had clogged it up. he cleaned it all out and fixed the heat sync and it worked fine for around 3 months and it started happening again! I clean my computer out every few days to ensure there is no significant build up. he had another look and decided to replace one of the fans with a better one, however this never resolved the issue.

ive began thinking my video card may be damaged causing the lag (just speculating, i dont know much about computers).

computer OS: windows XP home edition 5.1 build 2600
processor: intel(R) Pentium(R) 4 cpu 3.40GHz (2CPUs)
memory: 1024mb RAM
Video card: RADEON 9600 XT-4 (256mb memory)

game requirements:
-512 MB RAM
- 1Ghz CPU
- For DirectX or OpenGL graphics: 128MB 3D Graphics Card (i.e. Nvidia® GeForce™ 4 or above, or ATI 9800 or above)

any advice on how to check if my card is damaged? or does my computer simply not meet the specifications? if any more information is needed please let me know and how to access it.

thanks in advance!