wow you have the same problem as me... normally my computer stays at 40-60degrees, though when i render or go to a gaming LAN it tends to go to 70-80degrees which then just freezes or turns off for a couple of hours

i say get a new case like i will be doing in the next week... im going to try get internal water cooling case though they are abit on the expensive side but its what i need, soo yeah

my specs:

AMD Athlon 64Bit 3000+
ATI Radeon 9800 PRO 512Mb