this sounds very much like the main pc powersupply - i had one do the same - ended up buying a new 10 quid power supply for the case from microdirect and all was fine - it would be ok until it tryed to do any work and loaded up the powersupply , so on first startup , it could reboot say 15-20 times if u left it to it before it finally got into windows, then u would launch something that made the cpu work for a few seconds and "click" reboot again.