Subject says it all. My PSU's fan is not speeding up and stays a visibly slow speed. It almost looks as if it hiccups (speeds up for a fraction of a second then slows down, speeds up slows down)
I've cleaned the PC inside and out of dust and such. Little to no dust build up inside the PSU.
You power supply fan may be thermally responsive, it only speeds up when it detects that the air moving thru the unit is hotter, therefore increasing system cooling. In this instance there isn't anything you can do, it will just adjust the fan speed as it sees fit. The 'hiccups' relate to the fact that there is no dampening on the thermal response on the fan, therefore it enters a condition common to mechanical governors called 'searching' where it will overshoot and undershoot the 'sweet spot' that provides proper cooling at minimum sound output. Additionally, there is not much you can do about this as it is simply an oversight on cooler master's design of the thermal regulating chip.
Either way, your power supply is operating normally.
This wouldn't be because of your power supply. The most likely problem is if your computer has built up allot of dust over the years; open it up and vacuum out all the dust that you can spot, but make sure to get in between the fins of the heat sink on your processor and your graphics card. The other possibility is that your thermal paste has dried out between the processor and its heat sink; which is likely if you left on the factory stuff or if it has been a few years from when you put it on.
If getting the dust out does not work, then I'd reapply new thermal paste; and use a good brand such as Arctic Silver 5.