Only Stable Whilst Underclocked

Ok basically the problem is my system is only stable if i underclock the cpu bus from 166 to 165, im running pc2700 ram and ive ran memtest for 7 passes and no errors, i have 2x256 sticks i have tried running 1 at a time but the system is still unstable, under prime 95s torture test my cpu never goes above 50c and is around 41c idle, i am unable to play any games at the cpus proper speed but when i underclock games run perfectly also i have been running prime95 for a few hours now and no errors whilst underclocked, when the cpu is set to its proper speed prime 95 gets an error after between 4 mins and 1 hour usually 4 minutes, ive updated my bios tried different os, just upgarded my psu to a 550 watt, tried an old hard drive,cleaned with compressed air, nothing seems to work! please help! btw im running the latest version of prime95

ps- im not sure if i can change memory timings in the bios it has a cas latency option but i am unsure what this means plus memtest shows the ram to be good, so is this a motherboard or a cpu problem or heat problem?

This may sounds odd but try bumping up your AGP/PCI lock a bit. Try setting PCI clock at ~34-36 instead of 33.

Your CPU is under-volted, raise the Vcore a notch. Also, 50’C seems a bit hot for stock clock. Running my 2600 Athlon at 205x11 with 1.775 Vcore and it never breaks 55’ under full sustained load, 50’ is more common under load here.

You don’t say that happens when you play a game at 166, if it reboots then you might have a BIOS setting for thermal resets of some such thing. Re-seating your cooler, or a better cooler will improve stability too. Cool = stable.

But I’d start with more Vcore, many boards will under-volt the Athlons.