Computer Running weird.

[PCN00B]

I have a prebuilt system that wants to seemingly over heat when I stand it up straight, but if I start it up lying down no problems. I've touched the surrounding area and CPU is an athlon 2 but dosent feel hot...I took off heatsink and reapplied with out new thermal paste, I also unplugged some of fans because I was trying to remove hard drive but plugged them all back and reinstalled heatsink just fine. Anyone got any ideas why fans kick up to high gear and turns off? 

[PCN00B]

Heatsink wasn't properly reinstalled duhh

 

[Joe4096]

It could be a weird fan curve. But download something like a temp monitor. OR try and get something like A Hyper 212 EVO if there is clearance.

[tekgeek1205]

if you left the old TIM on the cpu and heat sink and dont have any more it might be best to clean the old off. it could be causing an uneaven mount. Whats you heat sink?