computer locks up and sometimes restarts

i have a asus p5n32 e sli mobo with intel core2 duo 3.00 cpu 535w psu with for fans running at 2000rpm. when i wake up pc run for about 1.5 hours then freezes on me or restarts then after the restart or me manually restart (cause it froze) it’ll become more frequent. today i noticed like right now that the cpu is running at 155degrees fareinheit, gpu (xfx gts 250 1gb) is running at 130, mobo 116, this all started about 4 days ago out of the blues, new cant play games or anything for long. could someone tell me what is broken? So far i have replaced the thermalpaste on the cpu (which seemed alittle thin), yet cpu temp is at 161 now, i do know its an over heating issue, just dont know what is causing it.