98 Dakota Oil Pressure

I just picked up a '98 Dakota with a 3.9 V6 that I knew needed a little work but now the oil pressure is acting up. The oil pressure would stay at just below 40 and then when the engine got hot it would suddenly drop to 0, then jump back up, then jump back down, so on and so forth until eventually it would stay at 0. If I shut the truck off and restarted it the oil pressure would go up fine but do the same thing again. So I changed the oil and the oil pressure goes up to where it's supposed to but slowly goes down to just below 40 after it's warmed up, and eventually does the jumping to and from 0 thing. I replaced the sending unit, and it didn't fix the problem. It doesn't do this if I let it idle from start or if I drive it VERY lightly. Aslo if I drive it with no oil pressure the engine doesn't make any noises and it doesn't lose power. I tried to get a mechanical oil pressure guage but no one has one, and there are no mechanics in the area that won't charge to check it for me. My cousin and my freinds dad (definately not mechanics, but both are very knowledgeable) both came to the conclusion that it's electrical but I have a hard time believing them. Can anyone help?

Thanks

P.S. Sorry for the long post

I had the same problem with my 2000 v6 4wd but mine sat for about 1 year. Take out the oil sending switch and get a fitting the same size. Put a rubber hose on the other side of the fitting, then a guage that goes up to 100 lbs and attach that to the hose. Use hose clamps. Screw fitting into oil switch, start truck and keep an eye on the guage. if it stays in the normal range then its electrical. if it drops like mine did, then you need to change the oil pump or clean the pickup tube and screen.