Voltage Fluctuations with Motor Running

[Dreyvius]

Got a 92' 240 fastback with sr20 redtop, car runs fine. Only when the motor is running all the lights are fluctuating in brightness, even the dash digital clock, and power windows. I'm assuming the voltage regulator on the alternator is the cause, I currently dont have any testing equipment.

Any other ideas as to the causes for such a problem?

I'm not good with electrical stuff really, got the car from a friend. He didnt know why its happening either. I can't find any loose grounds so far. The only thing doesnt fluctuate is the Greddy boost controller. Currently the car doesnt have a deck or amp hooked up.