Engine Oil Pressure Gauge Problems

It is either a oil pressure gauge or a wiring problem. Since you have already replaced the sender, you get to chase the wiring as a first step. If you aren't good at electrical troubleshooting, this will not be an easy fix. The phrase "my mechanic" tells me that you probably aren't good at mechanical/electrical troubleshooting. These cars are not a good choice for someone who has no mechanical/electrical skills. Due to their age and previous owners efforts, they require lots of work and tinkering.

If you tell me that you can troubleshoot electrical circuits at a high skill level, I can help you find and fix the problem. If you aren't it will become evident very quickly, so don't exaggerate you abilities.