Processor Temps:sound Comes And My Pc Turns Off

my processor is a core 2 duo E4600 @ 2.4Ghz.The problem is that sometimes when i switch my pc on the beeping overheat sound comes and my pc turns off. sometimes when i do manage to turn it on, i use coretemp and speedfan for the temp. reading and in the beginning it reads at 83 degrees celcius! then it cools down and the temp. remains betwwen 65c - 70c. yesterday the nomal temp was between 40c-45c. the sudden increase in its normal temp scared me. this increase happenned after i cleaned and dusted the insides of my pc. while dusting it i didnt do anything near the processor. i have 3 8mm fans in the pc. 1 blowing on the processor and 2 sucking air out-one from the back and one at the side.