1992 Plymouth Grand Voyager le 3.3 about 170,000 miles

I have not been able to figure this out. This 3.3 was running perfectly, well maintained and suddenly I heard what sounded like a fairly loud tappet like knock. Without tearing this machine open, I thought it was possibly a valve lifter. Also, shortly thereafter, while on the highway my oil pressure gage suddenly dropped to zero. I have changed the oil pump (although there was nothing wrong with the old one, and installled 2 different aftermarket oil sending units. The wire that goes into the plug for the sending unit seems to be "stretched" a bit, but otherwise appears intact. The engine also sounds like oil is not getting to the top of the engine, but I can't be sure. I'm afraid to fire it up as it's been sitting for some time now. I suspect a clogged oil port or a bad main bearing, but it could as well be something really simple and inexpensive. I'm not a true mechanic. Any thoughts would be helpful. I took it once to a mitsubishi dealership, where they listened with a stethoscope, and they merely said it was a "weak engine". I suspect it's not, thanks, kevin c