S3 standby stopped working

[User1001]

I have been using the S3 state of standby for about a year and for some reason this week the computer wouldn't resume from the S3 state and would just restart. I didn't install anything new (hardware or software). I was due for a clean install anyway so I backed up my data and did a clean install of Win XP home edition with the same results. Any ideas on how to fix this? Thanks in advance.

System:
3.0C Ghz P4 800 mhz fsb
1 GB Corsair XMS DDR 400
Geforce FX 5800
Gigabyte 8KNXP motherboard (875 chipset)
Western Digital 36.4 GB 10k RPM Raptor
350 Watt Antec PSU
Antec Sonata Case