2001 astro electrical problems

I have a 2001 astro which has a voltage drop , when you back up and put the vehicle in drive right before you take off the voltage drops to about 9 volts, the dash lights go dim, and theres a loss of power at the accelerator, this causes the driver to push on the accelerater more the the volts goes to 14 v and everthing is back top normal. this also happens when you are driving under normal conditions. any ideas?

I would have the alternator tested by a repuitable tech. I had a 79 Caddy that would die while you were driving. Would not start, until it sat for a couple days. Then it would start right up and the process would start all over again. Finally changed the Alternator and never did it again. Not saying this will fix your problem, but it is a start.

Something is draning the power while it is under load.
Is the airconditioner running when this happens?