2000 Oldsmobile Alero My 2000 Alero fails within 10 mins of

My car is a 2000 Olds Alero, 2.4L, 4 Cylinder. Auto trans.
When I first start my car and drive, as long as the engine is cold, has no problems. Within 5-15 minutes of running I run into some problems. Normal operating temp is mostly where my problems occur. Temp never goes over the middle, no overheating.
The problem I'm having is very low/loping idle (600-900rpms), and when in gear, rarely but sometimes also in park, the car hesitates and wants to stall out. It starts right back up though most of the time. I have to press the gas while turning the engine to make it start up smoothly. Otherwise it sometimes catches itself but idles very rough, rumbling. Revving up eliminates that.

Normal RPMs for being in gear during cold engine is about 1000 - 1300 rpms. Once its been running a few minutes, as engine temp climbs to normal problems get progressively worse. When driving the car hesitates and tries to stall when my rpms are around 1100 - 2000. Steady acceleration is fine, no stalling over 2000rpms, no highway stalls, no mid driving stalls. I can press the accelerator and rev high to push through most choking and rumbling and theres a slight delay then car catches and drives fine. Only low speed, low rpms cause engine failure. Like redlights, stop signs, in bad traffic, etc. Anything over 20mphs runs like new.

Have replaced spark plugs, coil housing, fuel injectors, fuel filter, catalytic converter, IAC valve, air filter, fresh oil change and filter. Used 4 bottles of fuel cleaner over 3 weeks. Replaced an O2 sensor a few months ago when having similar symptoms, seemed to temporarily fix problem. Checked vacuum hoses over and over, every way except with a guage. No cracks and all firmly attached that are accessible from checking topside. Checking fuel pressure today if I can figure out how to use guage (car has specifics for this test).


SVS, ABS, TRAC OFF are on and ding when they come on (from off to driving 30 feet comes on), Check engine light is always on. Did an OBD diagnostic. P0420 catalytic threshhold below efficiency (we just replaced 3 months ago, had to cut exhaust and use 3 C clamps to attach). Also getting a code saying I'm running too rich in my fuel mixture. (I forget the code)

One other thing is a small buildup of carbon deposits in exhaust pipe past CatConverter, rattles occasionally when revving and letting off the gas. Spits out little black drops of residue occasionally.
Had a slight constant oil leak the past few months, oil stabilizer seems to have fixed that at least for now. Was going through appx. 3 qts of oil every 2-4 weeks. Have not needed oil for over a month now. Coolant is full, full power steering fluid, brake fluid full. I did notice the other day after driving appx 30+ minutes when I turned the car off, I could hear a boiling sound coming from the passenger side under the hood. Went away after about 10 seconds or so.

ANY and all help is very much appreciated! If you need more info I can provide it.

Me and my stepdad are both kinda baffled by this problem.

UPDATE! Went to Advance Auto Parts and ran another OBD diag.

Info is as follows:

Codes
P0172 System Too Rich Bank 1
P0300 Random/Multiple Cylinder Misfire
P0420 Catalytic Efficiency Below Threshhold Bank 1

Freeze Data
Cause P0172
RPM 1962
Load Value 68.2%
MAP Sensor 62KPA
Coolant Temp 167F
Short Term Fuel Trim 1 -5.1%
Long Term Fuel Trim 1 -8.0%
Short Term Fuel Trim 3 -42.4%
Long Term Fuel Trim 3 -30.1%
Vehicle Speed 39mph
Fuel System 1 Closed
Fuel System 2 N/A

Side note? Oxygen Sensor Monitor INC
the rest are N/A or OK