1990 pontiac bonneville le 3.8 liter v6

car has 252k miles runs ruff after engine warms up, looses power, if I give more gas, car will sputter,
but rev up to 5k engine smoothes out. this situation constantly comes and goes. Help!!!

Hate to say it but I think the cheapest in the long run is to get it on a diagnostic computer to see what the sensors are saying. If it runs fine cold, but when it goes into closed loop operation, that means the computer is now taking readings from the sensors to set the fuel ratio etc. Until closed loop, it is operating off of pre-set conditions. Could be something simple like a bad o2, engine temp, air, sensor etc. Or something more serious like weak coil. Without throwing parts at it or doing some diagnostics yourself such as testing for fuel pressure and spark, its hard to say.

I agree; without more information it’s near impossible to narrow it down.
You might check the fuel pressure regulator. If the diaphragm has ruptured it could be pulling raw gasoline into the engine while it’s running.

A cold engine may burn that extra fuel but a warm engine will run rough because of it and higher RPMS means the throttle plate is open enough that the engine could be consuming all of that extra fuel. (Not saying that it’s right even if smooth.)
You could try pulling the hose off of the regulator and inspecting it to see if gasoline or gasoline vapors are present in the hose.

That’s strictly a wild, wild guess at this point.

Lol I had one of these cars given to me a few years ago and it had been driven with water in the oil and slipped a rod bearing… The crank was wore bad so my father and I took and cut the piston rod off to make two “caps” and put that back on with oversized bearings in it … we took and welded the piston into the cylinder to keep oil from somehow going out the exhaust and then disconnected the fuel injector for that cylinder … it ran fine at low and high rpms but mid range was pretty rough