If your fans crank to 100% and make a lot of noise and then you receive a black screen or screen flickering, then your GPU is failing. Why is it failing? It could be a few different things. The worst of all being that you PSU failed. This GTX 750 only requires a 300w, and your PSU is old, that could be the problem, but there are other things you can try first.
I recommend opening your case, and re-seating your RAM and GPU back into their slots. Take the end of a pencil and use the eraser to gently wipe the gold chips on the end of them. Make sure everything is clean and use compressed air, or simply blow your PCIE slots.
Sometimes something as simple as re-seating the cards and RAM can fix this issue.