Of course, I'm sure you checked your oil level. But, when is it occuring? Are you driving down the street, or is it at a steep incline or decline when off roading? If #2 it could be that the oil sloshes away from the pickup tube and that will account for the drop in pressure and decrease in RPM could be caused by oil flowing into cylinders and around rotating assembly during incline. Or could be caused by a faulty ECM, if RPM drops, oil pump is driven under load from engine-> your drop in oil pressure.
If there's a will, usually there's a way, but then you get to that bolt that you still can't get to with two extensions, two universal sockets, and a contorsionist midget trying to help you.
1999 XJ Four door, mostly stock for now, but I am in the process of building a custom plate bumper.