Need help! 95 Pont Transport

I really need help! We have a 1995 pontiac transport se and I would like some suggestions with the problems we are having. When it is idling the voltage drops and then when we move the voltage goes up as well as the wipers work when they want to we usually have to keep hitting the wiper arm to get it working the wiper motor was replaced when we first got the van. Another problem is that when we travel on the hiway when the gas tank reaches about a quarter tank then it stalls out and we have to open the gas tank and let the pressure out of the tank. Is ther also a way to test the block heater to see if it works other then just plugging it in and listening for something. I would be very grateful if someone could be able to answer my questions.