What could be the cause, has anyone else experienced this problem?

What is your question?

I was driving my 2000 dodge caravan sport out of town and all of a sudden my dash lights all came on and started flickering and my headlights went dim and the van died. Took battery off and went to auto store only to have to buy a new, charged battery. Vehicle ran fine for about 15 minutes then did it all over again. So we then took alternator off and had it tested to find out it was bad so we replaced it with an alternator that tested to be good before we put it on. Cranked up and ran just fine for an hour. Then it started acting even worse. All my gauges seemed to had quit working, my headlights went out, every dash light came on and even when I would push the had it would not exceed 35 mph. Now it's sitting on side of road. What would my problem be??