Trans-port

hi i have a 1998 transport just recently the gas gauge has been erratic in reading the tank contents whenever i filled up the tank the gauge read of scale and after 350kms i still showed of scale sometimes when i turn the van on the gauge reads half full but after 5 minutes its up to full then it will go back down to three quarter full its up and down like a qo qo the dealer tells me the full pump the contents sender is all one unit i can replace it for a total cost of $1100 canadian anyone have any suggestion as to the problem

ceheck your ground wire back at the tank harness.