Question about car, oil pressure dropping off

Hey guys, I have a little problem, I did a tune up like a month ago, plugs wires, cap, rotor. Car was running perfect. And about a week or two ago my car starts acting funny. When I come up to a light, it starts surging as im slowing down, surging bad, and the oil pressure gauge goes down with the rpms, the battery meter also goes down with the rpm's, battery is new though, I know if the battery was dieing it would take the oil gauge down with it as it surges, it happened before to me. But this time its different. It usually starts to act up after I drive it a couple miles.

Today I drove it down the highway for a good half hour, by the time I got back to the city i pulled up to a light and it starts doing the surges again. Then when I get to the next light I hold the gas alitle bit to keep it steady and I slowly let go, and now the car will idle perfect at 800 but the oil pressure gauge drops past the red line and stays there, when I hit the gas or start moving itll go back up. The car runs fine, pulls real hard, no notice in performance loss, its just the surging and pressure drop when I come to a stop, and when I pull into a parking space itll shut off before I get a chance to straighten the wheels and put my windows back up, itll surge real hard like once or twice and die.

What do you guys think? When I got home after my trip today I opened up the oil filler cap and just a little smoke was coming out of it, I noticed dark oil on the filler cap, it was a little wavy textured but was dark, wasen't milkshake colored. Not over heating or anything. Other than that I didnt notice anything different in the engine compartment, everything looked fine. Help me out! Guess its time for an oil change real soon, sucks I live in an apartment complex not something I can really do.