1990 Lebaron idles very rough

My 1990 Lebaron (3.0 L V6 automatic) recently developed several fuel
problems. About 2 months ago the car was working fine. Then I went
to start it one afternoon and it was very difficult to start. I drove
it to work and back (about 45 minutes highway/city driving) several
days. The only other problem appeared to be that at mid to high
speeds, it would sometimes buck slightly like there was a transmission
problem. On the way to driving it to my mechanic, the problem got
much worse and the engine got rough. It nearly stalled out a couple
of times trying to accelerate from stop at traffic lights. The
roughness was there in park & neutral as well as drive.

The mechanic replaced the fuel pressure regulator (cause of the
problem) and changed the oxygen sensor and did an oil change. The car
seemed to be ok again. I drove it for a few days, and the bucking
problem at high speeds returned. The problem quickly got worse. Now
the car is very difficult to start again, and it will completely stall
out if I accelerate much at all. It usually idles very rough in park.
I tried changing the oxygen sensor (to a good one that was pulled at
50k miles), spark plugs, and spark plug wires with no improvement or
change in symptoms. I've also changed the following items within the
last year: distributor cap & rotor, fuel pump (in-tank), fuel filter,
air filter. Through this whole ordeal, the computer diagnostic code
continues to show code 55 (all ok).

Any ideas as to what's happening would be much appreciated.