Pressure is not steady go up downs. - Garden

bad storage tank or check valve ,you may be have water running back down well. or a bad pressure switch. You should have a pressure gauge on tank or line is it holding steady or falling or keep rising every time pump runs.

Sounds like wiring problems with the engine harness.
The oil pressure not working could cause the hard starting problem.
The idle problem could be a faulty idle air control valve or a vac leak.
I would start with the check engine light and find out what the codes are.

Did you try changing the actual oil pressure gauge?

try keeping steady pressure on shift handle while moving to disengage fully then try again. sometimes it takes awhile. I had a
heck of a time with mine until i kept steady pressure on the shifter
to get it out of 4 wheel drive while moving,, gentle pressure.

OIL PRESSURE GAUGE SHOULD HAVE STEADY READING.IF FLUATING I WOULD HAVE OIL PRESSURE CHECK SOUND LIKE FAULTY OIL PRESSURE REGULATOR.

If it is leaking as steady as you say when the engine is running, it could be coming from several places you can check. Raise and block the vehicle good and steady enough to where you can slide under it with a good strong light. Check the oil drain plug, pan gasket, oil filter may have a hole in it from a rock flying up, the sending unit should be on the side of the block. It could be the rear main seal, but Its doubtful as you say its a steady stream when its running. You shouldn't have a problem finding it if its a steady leak, you will just have to look where its running down with the engine on. Keep me posted, and good luck.
Shastalaker7

Hi sounds like you should change the pressure switch

Check the fuel pressure at the shredder valve, you should have 13 to 13.5psi, anything other then that will give you problems in the GM TBI. If the pressure is bad, then you may have to replace the fuel pressure regulator at the bottom of the TB. 
Good luck and keep me posted. 

change the sparkplug

might be ur throttle position sensor