low oil pressure/ruff idle

Hi everyone I'm new to this so I hope I'm doing this right.

I have a 94 cavalier 3.1 liter Z24. When I start it up when its cold it seems to idle ok and runs between 40 and 55 pounds of oil pressure. But when it gets at normal driving temperature and you pull up to a stop sign or red light it idles ruff and oil pressure drops to about 15 pounds if in park and if in drive drops to about 5 to 10 pounds of oil pressure. But it doesn't knock like its staving for oil, it just runs real ruff. I ran the computer codes on it and it give a fuel air mixture valve that has to do with the idle and a oxygen sensor. Both have been replaced and computer codes are clear now but it still has the same problem. Also sometimes when I go to start it, it turns over fine but it want start so I just leave it till the next day and it starts fine. It has a new battery. Don't know if this is part of the same problem or something totally different. Anyone got any ideas I'm totally stumped.

Found out ignition switch bad. This problem has been solved. Other problem still there. These things have been fixed or checked by my mechanic to eliminate what he thinks ma be wrong.
1) Compression check
2) New plugs and plug wires
3)Checked for vacuum leakes
4)Voltage to fuel injectors
and a few other things I have forgoten what they are. Still trying to solve the problem. Still need help.


Thanks Fiserman