Please help to narrow down the problem in my 1995 Chevy Lumina 3.1 engine

Could be any number of things. Vacuum leak sounds most likely along with a fuel pump that's losing pressure over night.

The idle air control could be causing the extra air as well it may not be a vacuum leak.

oil leak could be a number of seals off the block and it bad idling could be caused by bad o2 sensor or throttle positioning sensor

Ok been jerked around quite a bit trying to find issue, took to mechanic, who told me it was a relay and rcv sensor, but this mechanic didnt do anything except fix the leaks, and now say intake manifold is leaking, he wrote things on receipt saying he repaired fuel injectors, did a engine flush, and winterized my vehicle, the car still had problem starting, but when I started the vehicle the rpm was no longer going up and down, drove the car for over an hour, came out the next morning the same problem still there. Difficult to start, have to give gas and hold key in start position till car starts, and rpm still going crazy. Discovered on the day I picked up the car, the mechanic broke into my gas cap, that has a lock on it, he claimed he put gas in it $10 to be exact, but there wasn't a drop of gas in it, Im guessing he poured fuel injector cleaner in my gas tank to cause temporary fix would last long enough to get my $350 he appears to be a fraud according to other reviews online, should have done more research. Took the car for second opinion yesterday, was told that after putting the car on machine, that the o2 sensor has gone out, and that the fuel pump is going out, mechanic also did a test with a gauge under the hood testing the fuel pump pressure, it was about at a 9-11 told me it should be in the 40's and eventually my car would no longer start, told me to turn my car in the on position four to five times before starting the car. I believe this is the problem, and thanks John and David appears you may be on the right track, if mechanic is right. I took the car back to the place that previously done the work, and claimed that did diagnostic to find out if he checked the o2 sensor and fuel pump, the mechanic wouldn't even come out to talk to me, sent his wife he told me my car is not worth fixing, and its very old, it is old a 1995, but only has 92,000 miles on it, she said I was better off buying another car, and said fixing fuel pump would risk other problems to begin with other parts, and it will not stop. Ok so what do you guys think, should I attempt to get these fixed to see if it resolves the problem, the car runs beautiful once it heats up, good heat, power everything. Just unsure of what to do, and sick of being taken advantage of ,please help.

Sounds like what happened to mine. It quickly was turning into a money pit, at the mechanic every 3 months with only 77k miles, including 2 p/s pumps in consecutive years, pressure hose, rack and pinion. Required a new O2 sensor every 10k-15k miles. I spent an additional $1000 fixing it (injectors, O2 sensor, IAC, etc) so it would be sellable, and it still needed about another $600 to fix fuel pump. The fuel pump repair requires taking out the gas tank. At the same time, the A/C went out and was quoted another $500. Valves lifters were starting to tick at only 77k miles, would cost about $700 to fix. The car needed about another $500 in new tires, so adding everything up the math didn't work for me, since I was still going to have a 15 year old car that wasn't worth the money needed to repair it. IMO, it depends if you could afford or not a new car. When they're good, they're good. But when they turn sour, they're sour. Don't place your money thinking nothing else will go wrong.

BTW, budget in changing the infamous manifold gasket if you haven't done it already.

I think is a fuel related problem..