computer freezing help

i have a AMD Athlon(tm) 64 Processor 3200+ 2.21 GHz , 1gb ram (2 512 dimm sticks) ,Asus k8n mb , windows XP pro sp3(had sp2 in and done the same thing) ATI Radeon 9550 , 2 harddrives 1 120gig 1 200gig both maxtor (dont see these being the problem but thought id add them anyway) 1000w PSU.

ok now to tell u what is happening every time i try and do something like run a game my pc locks up sometimes almost immediatly or can run well for a few hours before freezing, i have recently bought a new cpu so its definetly not that(i thought it was cause i had overclocked my system some time ago) psu has all voltages working fine (lol it should do :P its 1000w :) )i have a strong case fan on the rear wich is keeping my M/B quite cool currently my psu is running at 20c and m/b at 27...so i dont think its a overheating issue.
any ideas on what could be causing this would be greatly appreciated thanks :)

m/b temp 20 c and cpu** 27 c i ment lol

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