I have a problem with the signal going to my monitor. When I power up all is fine monitor comes on then after about 30secs it powers down. Tried using another monitor and different cable but get the same, so get the feeling its something in the pc. Any ideas!!

Check the graphics card is seated properly in the computer.

If you remove it, and put it back, it may just clean any poor contacts.

Assuming of course yours is not part of motherboard.

If Fingees solution does not work - and don't forget to check both ends of the monitor cable as many are detachable - then see if you can boot into safe mode without it freezing.