Why is my oil pressure gauge not working right?

I have a 1989 Chevrolet Silverado K1500 4x4 with a freshly completely rebuilt 350 (tbi) that has a .030 bore. I had this problem before I rebuilt the engine, and I assumed the problem when I found the oil pressure sensor (for the gauge) under the distributor had a broken fitting from the previous owner trying to fix that engine (i will not go into that lol) but I got a new fitting and sensor installed it with teflon thread sealer and all that such. The engine has a new oil pump that I know works and works well and was installed properly, I know the shaft from the distributor to the oil pump is installed properly and has the small plastic piece installed properly on that also. I know the oil sensor next to the oil filter is also good. I checked all wiring connectors. I currently now have 213 grandma driven miles on the engine, now here's whats going on, when I first started the engine the pressure was between 30 & 60 and stayed close to 60 when out on the highway and close to 30 when stopped or moving at a slow speed. Now when i start it, it is at about 15 the engine runs for a few mins then nothing but a "check gauges" dummy light it shows no pressure. What could be causing this problem im kinda at a dead end tryin to figure this out? Can I get a mechanical gauge to install under the hood?