Gas Gauge Problem

[ryguy2950]

Ok so I filled up my svx for the first time...i just got it yesterday with 200k miles...it runs great and this is the only problem... i filled it up and the gas gauge will tell me how much i have in there by sitting the car still while running so i can let the gauge climb to the actual amount. Then once i get going the gauge goes down to empty until i stop again then it will go back up...(unless the car is off) then it wont move at all of course) if anyone knows how to fix this problem........plz help