Install the gauge at the engine. If you install it at the gauge, you are not eliminating all elements of the gauge that could be giving trouble.

Yes, at least on earlier engines thare are some over pressure relief valves. They are nothing more than a spring and it would be very unlikely that the spring would get weak and allow pressure to be dumped that far too low.

In the earlier engines, this relief valve was at the front of the engine in the timing chain area. There was a plug on the outside that you removed to get a socket onto the valve itself so you can remove it. You need to get some detailed prints on your engine to determine where it is.

Good luck,