Can a weak or faulty power supply unit cause graphics card to overheat?

Your PSU won't cause the GPU to overheat directly, but could be the reason indirectly. 105 is really hot for a GPU (graphics processing unit) but the greatest reason is lack of airflow in the case. There should be at least two fans in your case, one to draw the air in, usually from the front, and one in the rear to push it back out. Many systems have 4, 5, 6 just to make sure there is good air movement. Also the GPU heatsink could be jammed with dust and other debris causing this problem. But the biggest reason is likely that you are overdrawing your PSU causing it to generate tons of heat. A basic system with a 4650 runs about 150w, a weak PSU that runs at 70% inefficiency (which is not unrealistic for a generic PSU) maxes out at around 190w, making it work hard to power your system. I suggest get some canned compressed air, blow out the GPU heatsink at the least but also change your PSU for something beefier.