Help! Jeep Oil pressure problem

I have basically the same symptoms on a 1997 GC 5.2l. The engine is new with about 2500 miles. Everything was fine until the last week or so. Now, when the engine is warmed up and idling in gear, the pressure gauge fluctuates and then falls to zero. Shifting to neutral raises the RPMs enough that the pressure rises to almost 30 pounds. I replaced the sending unit with no change. I've connected a mechanical gauge and it reads 18-20 pounds idling in gear, so I figure it must be electrical or gauge related, although I'm not thrilled with the idling pressure on a new engine. I'm planning to replace the gauge next unless someone has another idea.

I have the same problem with my 96 Grand Cherokee, I have been told to connect a manual oil pressure gauge to verify that it is not something electronic with the gauges. The oil pressure gauge on jeeps is computer controlled and we all know how reliable those are. I plan to try this next. If anyone out there has tried this solution, please let me know and save me some bruised knuckles. Thanks!

I think mine is fixed. I did connect a mechanical gauge to verify that I had oil pressure and would recommend it for peace of mind. But, I added another ground strap from the firewall to engine block and this seems to have solved it. So, I'd check grounds. Good luck, this was a fun one for me -- always like electrical problems.

When you connected the manual oil pressure gauge did you have any loss of pressure, or was the problem all electronic?

As I recall, the pressure with mechanical was a little lower than the factory gauge - before it fluttered and fell to zero. But since adding the strap, it's been solid.

When the factory gauge fluttered and fell to zero did the mechanical gauge say that you still had pressure? What sort of strap did you add to fix the problem?

I connected the mechanical gauge in place of the factory sending unit immediately after detecting the drop in pressure with the factory gauge. With the cheapo mechanical gauge, I never dropped below 20lb. I then reconnected the factory gauge/sending unit and again got the flutter and drop in pressure. So, it was pretty clear I had problems other than oil pressure. I replaced the sending unit with no change. I then got another gauge cluster from a salvage yard and it did the same thing. I had another strap identical to the factory stap that connects the block to the firewall, so I connected it as just a guess. I think the problem was the connection at the firewall - screw could have been tighter.I'd check those connections and if tight try adding another strap. I'd think any larger gauge wire would work. The factory gauge now never reads below 30lb at idle in gear.
I've also had the connections at the battery cause strange problems - as well as the battery itself. So, I'd start at the battery and load test it, check connections, check alternator output, belt etc.