Oil Leak

I am having trouble with an oil leak in my 91 Explorer. It will go from leaking barely at all to all the sudden leaking a lot (emphasis on a lot), and it has been going on for a while (months). I can't seem to find where to leak is coming from, although I think it is someplace high since I can find traces of oil behind the intake. I replaced the intake manifold gasket a few months ago, but I'm not exactly a car expert. Is there something I could have done wrong when putting everything back together? I also noticed that the oil pressure gauge will drop and fluctuate rapidly when the car is not moving (in any gear). The oil pressure returns to normal as soon as the car starts to move.

sounds like a lower intake or valve cover problem
as for the oil gauge check the oil level and add some engine treatment.

I heard that engine treatment is actually terrible for your car and that oil thickening addetives, engine treatment, etc. can be harmful to your vehilce.

if the oil gauge reads low at idle and raises on accel
there is an oil pressure problem. usually from clearances in the main bearings. the root cause could be plugged pick-up tube. engine treatment might prolong a costly repair.