I have an optiplex 990 & i have recently replaced the psu to an identical version after the last one broke but it has been weeks since that happened and things have been smooth until now,

my computer was running fine yesterday then i tried to launch it today but it wouldn't start, the pc had a blue light but the “3” amber LED was on soo i turned it off and tried to do some research but when i came back to start it again it seemed to be fixed and launched properly, i played on in for about 20 minutes then it completely shutdown as-if it lost power, then i tried to start it again and it had the same power & led display (also when i first turn it on some peripherals would start for lime a second then stop as the led display came up), then i tried a different power cable & socket and it worked again for another 20 minutes then crashed, then i had the same LED display, so i restarted it a-few times and it launched properly and i played for another 20 minutes or so and it crashed again, this happened a-few times.  

any ideas whats wrong?