iMod: Sounds like a loose connection.
St2b: Check your battery and alternator.
Both of you: Check all your grounds, clean the terminals on your battery, and make sure all of your power connectors are tight.
Most 15+ year old cars have had the alternator changed or rebuilt at least once. I just recently changed mine. It was the original Toyota part(from 1987). I'm using the alt. from a turbo mkiii(80 amp?) because I use electric fans, and have a small system in my car(300 watt amp, and two ten's). I don't have voltage drops or fluctuations. Charging system is steady as a rock at 14 volts. Doesn't even jiggle if I have the radio, a/c, and headlights on, and use my turn signals. It shouldn't, either.