I found out the hard way that the sending unit must be matched with the gauge. The gauges work on electrical resistance with the sending unit, which I believe can be affected by heat and ground. I’m sure there are plenty of EEs out there that will set me straight if I’m wrong. I chased a similar problem for a couple of months until I stumbled on the real cause. I should have used a mechanical gauge to diagnose the correctness of the pressure. Another ‘possible’ cause could be a stuck thermostat (but unlikely). Cheers, Rich