1994 Volvo 850 Volvo runs poorly after warming up

Engine Performance problem
1994 Volvo 850 Two Wheel Drive Automatic 145000 miles

The car runs great until it warms up. After driving for 10 to 15 miles it starts running poorly when accelerating. I will stop at a stop sign and then step on the gas and it will sputter, once it gets going it seems to take its time shifting into third and the rpms go up to about 4500. Usually if I let off the gas and slowly, and I mean slowly accelerate it will go but it takes forever to get up to speed. Here is a list of what has been done to the car:
new fuel pump/filter
new spark plugs
new impulse sensor
new air filter
new catalytic converter
new thermostat
We ran codes and came up with 2-1-4 which is engine speed sensor signal missing sporadically.
Would a tranny flush improve it?
We are completely stumped now.