Gauges on cluster are pegged/jumping

This is typical in GM based vehicles from 2003-2006. If your gas (Gas/Oil/Temperature) gauge is (pegged/jumping) and it never did that before sending it for repair, here are a few things that could possibly be the issue:

Faulty sensors may be at fault.
If you only have an oil pressure issue (flickering or going haywire or completely off), most likely it is the oil pressure switch/sensor at fault. It’s a commonly known defect along with clusters in the year range.
If you have incorrect fuel reading, be sure to check the fuel sender/sensor unit before sending it back in.

Once the fuel sensor or unit at fault has been repaired and replaced, the fuel gauge should display the correct amount of fuel. This can also be verified via a mechanic’s scan tool which can see the values of the gauge.