my 1993 astrovan speedometer and battery gauge jump back and forth while driving, what auses this?

Intermittent power to the gauges or a loose ground connection. It gives you something to watch on long trips to alleviate boredom.

Go easy there ten, you must take a hose to your battery, cleaning off the nasty corrosion, then wirebrush the terminals and any other loopy connectors, then look at the other end of the black wire make sure that it is attatched shiny surface to shiny surface...and check the black cable to engine too, as this is where it will draw those amps needed for a rapid start. Carefully assemble everything...and turn the key, drive around the block to find out it's a filter problem with the power going to the speedo...drat.