Monitor remains black/CLock Speed changing automatically

My PC configuration is as follows:AMD Phenom X6 1055T,MSI 880GMA-E45 motherboard,2*2 GB GSKILL 1600 Mhz DDR3 ram,GTX 470 with Cooler Master Extreme power 600 W power supply running windows 7 64 bit.

Of the late many times monitor remains black when switched on whereas I can hear computer beep and boot normally.After some fidgeting sometimes the monitor receives signal from GPU and starts.I read it on some forum that this happens when GPU does not get required heat.The solution was to keep the computer running and restarting it after sometime.This method has also worked sometimes.Recently I found through AMD Overdrive that my clock speed is changing automatically constantly for all the cores randomly, some increasing, some decreasing, some changing by nearly 2000 MHz.This happens when stated clockspeed by windows is 2800 Mhz.Otherwise computer automatically overclocks to 3458 Mhz.When I run my computer without GPU the clock speed remains stable.At that time too computer automatically gets overclocked to 3458 Mhz which is then forcibly clocked to factory settings.Please help me out.I am turning crazy here and Thanks in advance.