CPU Throttleing at 100%

My CPU is throttling at 100% i don't know why it jumped up all of a sudden but everything i do is laggy now. gaming is laggy and i can hardly watch videos on youtube even. i would really like some feed back on what i can do to fix this. on a side note whenever i overwork my cpu my comp will black screen and turn off no warning or anything. any ideas?

Operating System: Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.130708-1532)
Processor: Intel(R) Core(TM) i7-2670QM CPU @ 2.20GHz (8 CPUs), ~2.2GHz
Memory: 8192MB RAM
Available OS Memory: 8170MB
------
Card name: AMD Radeon HD 6900M Series

Thanks for any help given