Hep! New to Tahoe's and can't figure out what's wrong!

[Usmc2009]

2003 Tahoe fuel gage acting up, when I turn the vehicle on it reads the right amount and the trip meter is correct but then it will gradually continue dropping the come back up again as long as I'm moving, once I stop at a stop light or stop sign it drops to basically E most of the time as well as the trip gauge. If I go put $10 or $20 in the tank then all of the sudden I have right over half a tank again but I only bought 6 gallons? I just replaced the battery about a month ago and the fuel pump had been replace immediately prior to my purchasing of the vehicle in April. All my other gauges work so what could be the problem?

 

[bkboatnsleds]

Sending unit in fuel tank - but you have to replace pump too...