1985 Jeep Cherokee oil pressure problem

When I start my jeep I have around 40 pounds of pressure but after five minutes it goes down to almost zero. The motor doesnt make any ticking noises at all, but sometimes the vehicle makes a wierd noise when im on the gas pedal over 2000 rpms. I cant tell if its my exhaust because it is a little loose. What should I do is it my oil sending unit or what?
- also I have a leaking trans that I have replaced the gasket recently but began to leak again. Most of the time when I put it in reverse it doesnt engage right away, it takes about five seconds or more, this also happens in drive sometimes also. Could the noise be coming from my trans?