91 Silverado Problem. Need advice.

[luckydogger]

So here's whats going on. The battery went dead a few weeks ago. I put the battery on the charger and it wouldn't hold a charge. No problem, went and got a new battery. Put it in the the truck and it cranks good, idles fine, then when I take off down the road the gauges start jumping all over the place. Radio and AC start cutting in and out. The voltage gauge spikes all the way over when you hit the throttle. So I turn around and limp back to the house thinking my alternator is bad. Put a new one on and it didnt help. If anything its worse. It idle's pretty good in park but if you put in drive, the gauges start jumping and the engine starts idling rough. Can anyone give me an idea where to start looking for the problem?