ICAP

Not sure if this is in the right thread but here goes, I know a little about pc's but this problem has stumped me. On initial start up the computer can do either of the following: 1) Get to the start up screen (press del to enter set up) and it just reboots. 2) Get to the log into windows screen then reboots. 3) Get to the log into windows screen then just freezes. 4) Starts up no problem gets to the desk top, open programs then just freezes or reboots. This happens every time i switch the pc on, after about 5 attempts it suddenly works perfectly and it can be switched on and off all day without a problem, if its left for say 5 hours the problems start again. Any advice would be appreciated. CAP