You may have a bad cluster/oil gauge. If you take the gauge cluster out ( not very hard to do) there is a little white POS "reciever" ( the gauge itself) soldered to the circuit card. You can get them on Amazon or Ebay for cheap ~ $5.00. There are four solder joints to unsolder and it pops off the circuit card. I've had the same issue with my 2004 Pontiac Montana - speedometer, my 1997 Chevy S10 - temp, my 2003 Tahoe - oil and my 2001 Yukon XL - oil. TOnce its replaced you'll need to "adjust" the needle position. I have a mechanical aftermarket gauge I use connected to to the driver side bolt in the block near the front that I tapped on the YukonXL to verify the correct pressure and then set the cluster gauge needle depending on what it showed. On my 2001 YukonXL, I installed a Melling high output oil pump and when I verified the oil pressure. I was at 45 PSI cold where the cluster gauge was saying pegged; or you can order the whole cluster for ~ $250.00 and hope that fixes it. Also if ALL the gauges start fluxuating like crazy, there is the block ground behind the intake by the sending unit that is probablt not connected or is loose. Ive had that happen as well.