I had an older 83 GMC Jimmy K5, and it had some gauge problems too. After removing the gauge panel and all, I noticed the gauges made their connections via some metal clips that attch to pins ont he back of the gauges. Well I had a fuel gauge that was inaccurate. it would not show a full tank when there was one. I found out when taking apart the cluster, that the fuel gauge was not seated correctly in its pin sockets.
After i made that little adjustment, the gauge worked perfectly.
Perhaps it is just a loose gauge..
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.