PC Power Supply Issue?

Fitting a more powerful PSU shouldn't cause any issues. Unless it's faulty then it's more likely that you've disturbed something else whilst changing it. Check that all cables are plugged in properly and that you haven't missed any such as the 4 pin power plug on the motherboard. Also check that you haven't disturbed the hard drive cables or the power connection for the processor's cooling fan. If it still does it then try removing and refitting the memory and graphics card, and anything else that you can see. Check that you haven't somehow knocked the processor's heatsink out of position too.