gateway 7811fx cutting out

i bought a gateway 7811fx barebones unit on ebay. guy said he had replaced the mainboard with a good one from a seller he gets them from. said he couldnt get it to post with a p8400 chip. said it must be the power board in it. so i bought it, bought a power board and bought a p8400 chip for it. bought 2 x 2 gig chips ddr3 ram for the hp dv6 which are ddr3-8500 dimms. these are supposed to be compatible np. i first tried using a t9300 chip in it thinking it should work as they are similiar. well after messing with the power boards, i can use the original one that came with it or the new one i bought, dont matter, the system will cut out after running for a minute or less, but its always at different times. sometimes it will work for 3-5 mins, but rarely. whether i boot off a cd, a hard drive, whatever, it keeps doing it. when i put in the p8400 chip which is stock, same thing. im at a loss. hate to think its the mainboard. i have changed power supplies, or ran it off the brand new extended battery i bought for it. dont matter. it does it whether the battery is in it or out, running off battery power or off ac. and the battery will charge from the ac. what i do notice is when i runn it, and i also tried with hard drives in it or not, with dvd-rw or not, changing the ram around or not, dont matter. i found when i run it and its plugged in, the ,llights would be on, power and charge, but when it cuts out, both lights go out. then a second or 2 later, the charge light comes back on. almost like it resetted. anyone got any ideas? im stumped and pissed i got $600 into it. could have bought one at that price.

where is the thermal module on the 7811fx? i didnt see it. slow fan? yeah i wondered. i watch them both spin up, but the gpu fan dont really start back after that, and like i said 1 min or 30 secs or whatever and it shuts off completely