when i start up the van the oil pressure gauge needle will go all the way down to right and when i accelerate it goes back to normal. once my foot is off the gas,it will bounce all over the place. until i accelerate again. how can i fix the problem?

What year, what engine ?
IN any case, you can unplug the oil sender..which is the probable cause . THe 3.8 sender is proportional, the 3.1 is merely a switch that "fakes" a value for the needle . Both have a second contact that keeps the fuel pump running as redundancy to the fuel pump relay.
Faltering solder points in the instrument panel have been reported but usually make more than the oil pressure gauge go haywire.
Erratic voltages due to alternator failure are also cause of miscellaneous indicator freaks.

I have a '93 Lumina APV, I've had it since '98, almost 9 years, I got used to the bouncy oil pressure gauge. Sometimes it works fine, most of the time it is erratic.

Stefan