with a booting system....use hardware monitor...watch your power supply outputs. open the case check that all your fans are moving..even the power supply one. look at windows error log file.
http://windows.microsoft.com/en-us/windows/what-information-event-logs-event-viewer#1TC=windows-7
http://windows.microsoft.com/en-us/windows/open-event-viewer#1TC=windows-7
see if it calling out power issue or gpu issue. if it power issue i would start a rma on your power supply if it under warranty. at some point your going to have the part fail where your rig wont turn on. right now the part is starting to fail and it could be the power or the gpu. you have to try your gpu into another pc or try another gpu in your pc if someone you know has a test card to use.