Engine severe misfiring, stumbling

No idea what to try next.

(Prod. Mo. 1/92), automatic transmission, M50, non-VANOS, 243,000 miles (still usually 22-23 mpg)

Experiencing a horrible miss/stumbling pattern and I just cannot figure it out.

Started with an intermittent CEL back in the beginning of August. Code was O2 sensor. Noticed it had a slight miss every now and then, and mpg was dropping slightly over the next two weeks. Installed new O2 sensor. CEL has never reappeared, and no codes are present.

But the miss/stumbling has gotten absolutely horrible.

Following is a list of what I've changed/repaired/replaced, etc:

ECM
MAF
Spark plugs
Coil packs/boots
Injectors
Fuel pressure regulator
Fuel filter
Crank sensor
Cam sensor
TPS
ICV
ATS
Coolant temperature sensor
Feed/return fuel hoses (at the rail)
Two vacuum hoses from PCV

No vacuum leaks. Already removed, checked, replaced every vacuum hose, and even did the starting fluid spray test at various rpm's around said hoses, throttle body, intake manifold, etc.

Vehicle starts fine, idles and drives great, until it starts to warm up, then the idle becomes erratic and the missing starts. Driving at highway speeds (above sixty mph) and it is basically smooth, even when stomping it to the floor with the resulting gear range kick-down. City driving is nothing but stumbling, sputtering, chugging. It just seems like symptoms of intermittent spark and/or fuel. If I sit in the driveway with the parking brake on, my foot on the brake, put it in drive or reverse and press on the gas, it misses so badly it shakes the whole vehicle and will not get above 2,000 rpm's.
Any and all ideas are appreciated. Thanks!