Happens to me as well brother. I just replaced the battery on my 94 seville STS and it STILL does it. My guess is that a BIGGER battery would remedy the problem. However, nothing is wrong w/ your car, don't worry about it.
Back in my drag racing days.....
Sometimes I would just watch the drags.
I would usually stand at the finish line.
Of course the drags were at night.
I could always tell when the race started because the headlights would get brighter.
the head lights "should' get brighter with more RPM. With engine off the lights should be getting only about 12 volts if that because off loss through switches and wires, but with engine running and alternator generating output your voltage should be almost 14 volts (assuming no loss through switches and wires).
so my sugestion for finding out whats really going on is get a hold of a digital Volt meter.
1. take the negitive probe and ground it than check your batt volts first
2. turn head lights on with engine off and back probe the socket to find the volts of both high beam an low beam.
3. do the same now with engine running
4. do step 4 with some one holding throttle at a higher RPM