Engine cuts out... Sunbird 2.0L

The car:
1990 Pontiac Sunbird, 2.0L normally aspirated Inline 4, 189,000mi

The symptoms:
Engine cuts out / bogs above (estimate) 3000rpm. Throttle input must be done in gingerly increments, otherwise, engine cuts out / bogs. Car will not exceed 40mph on flat pavement due to engine power issues, uphill is worse. Car has issues on initial startup if it is facing uphill / downhill, once transmission is engaged the engine often dies.

Ok, I think that pretty much outlines the issues i'm having with this car. That being said, here are the Items I know are wrong or are not right.
>> Transmission TCC solenoid has sticking issue. Plug has allready been disconnected from the transmission....
>>Radiator fan was hard wired by previous owner. I am unsure as to why. When I chose to connect the radiator fan to power I use the default hot location the previous owner used which is on the ignition module... Connected or not, the above stated main problem does not change....
>>The incoming air temp sensor located on the air breather assembly is missing the required clip to hold it in place. It has been rigged with copper wire around it's flange to hold it into the air breather.

I have replaced:
Cap, rotor, plugs, plug wires, ICM. One full tank of 93 octane with fuel injector cleaner has been run thru the engine, no real marked improvment in current situation..

I was told that the fuel filter(s) have recently been replaced...

Based on what i'v said, has anyone got a firm idea as to what they think the cause may be? I'v got some suspicions, but I'd appriciate some educated input so as to avoid chasing my tail further.

Thank you in advance for your input. If you have any questions, or need more detail, please ask.

-Nathan

Did you replace the cat.? This sounds like a clogged cat, you may want to check it but with 189K it is on borrowed time. Also change the fuel filter, you may be starving the engine for fuel if it is blocked, even though you were told it was replaced it may have plugged again. One other thing check the fuel pressure with a guage. The fuel pump may not be giving you enough pressure to keep the engine running properly.

I'm not sure about the cat, it may be original, or maybe it's not.

I had a chance to work on the car this weekend and I re-checked the codes. Currently the only code coming up is 44 which indicates too much oxygen in the exhaust (lean). I bought a fuel pressure testing kit this weekend and gave it a go... Fuel pressure at idle is 11 to 12 psi, (Haynes manual says 9 to 14 psi on TBI engine is the norm) but when I goose the throttle and run the engine at higher rpm's the fuel pressure drops significantly, down to around 4 to 5 psi. Such a large drop in fuel pressure under load would seem to explain why my engine cuts out, it is starving for fuel.... How can I tell if this is the fuel pressure regulator, or the fuel pump. The manual is very vague with regards to the regulator...

with the car running at idle, disconnect the vacuum line and apply 15-20in of vacuum to the regulator with a hand vacuum pump. the pressure should drop as vacuum is applied, and recover when released. If not, the FPR is bad. If it passes the first test,reconnect the FPR vacuum supply line. As you rev the engine and the pressure starts to drop, pull the vacuum hose to the fpr. The pressure should spike to the high side of the range and stay up as RPM increases. If it doesn't, the fuel pump is at fault, as it cannot deliver enough volume of fuel.