Macbook pro running hot and freezing. Had to keep on an ac vent just to install osx. Please help!

ok, so i have a macbook pro its running hot i think. Previously i posted up that i was having trouble installing snow leopard.

I had a hard drive with lion on it i refused to wipe until i figured all this out. well the lion drive got so bad soon as i booted an opend up a thing or two
it would freeze and id have to reboot. it also was kinda warm
so i tried to install snow leopad after wiping it and formatting it correctly it would get almost done and freeze and id have to restart. after several attempts
i decided to set the macbook ontop the airvent on the floor and jack the ac on cold.
IT WORKED! Now i have a perfectly good working macbook! but it seems to only work on the airvent. once i start using it other places esp in my lap or on my desk it overheats and freezes. what is causing this? the fan is working! also i lost the screws to the bottom pannel, would that matter? i just have it attached with a few screws that hold the pannel on, not all screws are in.
please help.

Model Name: MacBook Pro

Model Identifier: MacBookPro5,4

Processor Name: Intel Core 2 Duo

Processor Speed: 2.53 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 3 MB

Memory: 8 GB

Bus Speed: 1.07 GHz

Boot ROM Version: MBP53.00AC.B03

SMC Version (system): 1.49f2

Serial Number (system): -----

Hardware UUID: 6AEED608-CE73-5897-A904-2F771E451F54

Sudden Motion Sensor:

State: Enabled