What PSU, Mobo, CPU and GPU, underpowered can produce consistent low FPS, a multi-rail PSU could cause problems and fluctuations, of one just headed south could also
A bad PSU could not drop FPS, but if the output isn't enough for your system then it could reduce your FPS from 60 to 0 by crashing your system (closing the game as well).
A GPU that isn't getting enough power will under perform (low FPS) and eventually cause some sort of crash. a CPU will crash much faster if not getting enough voltage and will probably BSOD or just shut off completley. Bad power supply will usually just cause a straight shut off with no BSOD or warning, and usually under load. but can certainly be sporadic and random. these things aren't fun to diagnose :[
What PSU, Mobo, CPU and GPU, underpowered can produce consistent low FPS, a multi-rail PSU could cause problems and fluctuations, of one just headed south could also