2000 Pontiac Montana that starts and runs fine for 7-12 min then looses throttle power. Sputters at 1k-2k for 10-15 min then runs fine for 7-15 min then runs even worse, wills sputter at 750+ rpm.

Check your fuel pressure, early 2000 rams are notorious for fuel pump issues. They start but run rough or lack power.
Were the lean codes for both banks 1 and 2? (P0171 and P0174)

the most common causes are vent solenoid and fuel cap
P0440 PONTIAC code possible causes

• Missing or loose fuel cap
• Incorrect fuel filler cap used
• Fuel filler cap remains open or fails to close
• Foreign matter caught in fuel filler cap
• EVAP canister or fuel tank leaks
• EVAP system hose leaking
• Fuel tank leaking
• Faulty vent solenoid
• Faulty EVAP purge switch

https://www.justanswer.com/pontiac/9xoy3-2000-pontiac-montana-3-4l-check-engine-light-p0440.html

https://www.yourmechanic.com/article/p0449-obd-ii-trouble-code-evaporative-emission-control-system-vent-valve-solenoid-circuit-malfunction-by-spencer-clayton

You may have a bad catalytic converter. They start coming apart and plug your exhaust. They cause all kinds of run problems.

check your harness...BUS LINE,from PCM TO THROTTLE BODY HARNESS..dont forget also your TPS..

Bad spark plug or wire. Observe it while running at night to see if spark is jumping.

possible vacuum line left off , loose air inlet boot or contaminated mass air flow sensor with whatever cleaner was used.

The buzz could be the fuel pump in the gas tank.
If it will run with the throttle open it could have an issue with the idle speed control valve or a dirty throttle body.
The computer controls idle speed by how much air enters the intake thru the valve.

Sounds like an electrical issue likely in the harness that runs from the body to the gas tank or close to the ECM and the wiring may be rubbing against the body close to the ECM and shorting out. Look for lumps in the wires or green corroded connectors.

check starter wiring for loose nuts, bad connetion. n what not. clean up terminals if needed n see if it turns over if not. check for power coming frm the yellow wire, or the smaller one. with the key held in start position, in no power you will need a starter solinoid. if you do have power go grab a starter n put it in,

First with your incorrect fuel reading, you may need to have your fuel level sending unit replaced. Next if your vehicle otherwise runs fine when cold, no sputtering or hesitation if going uphill then I would suggest that your check your vehicle's air intake/induction system Mainly your air filter to see if it is clogged up which would impede air flow into your combustion chamber causing poor throttle response or suffication (not having enough oxygen for your fuel/air mixture to burn. Next I would have a thorough check performed on the emissions systems, also including your o2 sensor and possibly even your catalytic convertor and exhaust system. Start with that and let me know what your results are and I can advise you further if problems persist or worsen.