Desktop keeps crashing - GT430

I have an Asrock H77M with i-3579K (dont ask - it was almost same price as non K) and 8gb ram powered by XFX 550W PSU. I have been having problems for past week or so in that if I use the GT430 then my games and sometimes the internet crash out and desktop restarts itself most of the time but sometimes I have to switch off and on to get it to work. I have gone back to older nvidia drivers with no difference. If I leave GT430 in its slot but plug monitor into MOBO (HD4000 graphics) then there is no problem. None of the games I would describe as graphics intensive - ANNO 1701 - Oblivion - C & C Generals - Age of Empires - TRS2004 and the likes. Anyone any ideas or am I correct in thinking the GT430 (5 years old) is asking to be replaced???