most important part is to make sure that your +5 voltage is reading at 5.10V at the back of the main board, and that the 3.3V voltage is exactly 3.30V. if the 3.3V is too low or too high, it will cause your game to freeze.
the CPU fan to my understanding utilize hardware monitoring, as in, if they're not spinning or working properly, the board will go into a shut down mode (not to be confused with it freezing). maybe it's not that sophisticated, but I do know from experience that if it's unplugged, the board won't boot up.
start with the power supply. the power plugs on the back of the main board have red, yellow, white and brown wiring.
red = 12V (irrelevant, unless you lack sound, which isn't the problem)
yellow = +5V (check that this is 5.10, that's a safe operating voltage)
brown = 3.3V (check that this is 3.30 volts exactly, this will make your game freeze if it's out of adjustment)
white = ground
not entirely sure why Sega have to be dicks and use non-standard wiring colors.
if you have one of the original power supplies, there won't be knobs to adjust the voltages, you have to use a teeny tiny phillips screwdriver, and be very subtle with your adjusting and make sure you don't slip with it and hit something inside the unit that can get shorted out.