My computer temperature very high then

computer works very fine at first then when i tried to open a video or open more than one url sites with big informations on it like youtube videos or any java or flash or much photos in it, i start to hear some voices from the computer case then computer turned off suddenly "not shut down" just turned off then when i turned it on again & enter the "press del to enter setup" & go to the place that see the temperature i found more that 95% 96% or even 97% of heat & it may turn off again after see it to 99% then if i waited for a time & turn it on i found heat became 70% ~ 80% but the processor cooler fan is working fine & i see it's speed about more than 2000 r/m & power supply fan too so i guess it's a cooling problem so how can i cool the computer it's intel pentium 3.2 Ghz with 2 G RAM & 1 G on board vega
& by the way i opened the case & put it in front of the air conditioner till it cam cool then saw the temperature of computer found the temp about 50% ~ 60 % & at the same time when i enter windows it still working fine & when surfing big url sites with videos or photos or javas or flashes all worked fine without hearing that voice & viewing youtube videos without any problems & the computer still working without any turn off suddenly but if i turned off the air conditioner, just few seconds & the same problem appears & voices lisened & it turn off, so how is the best cooling process or is there any setups from the BIOS to lower something that make the temperature became very high? & my processor cooler fan is LGA fan G4 "it's a circle fan with four long pins"