'93 Ranger low mpg, low power

[mtn93]

I have a '93 Ranger 4.0 4x4 automatic that I've been driving every day for over 15 years. The truck has 250,000 miles on it but the reman. engine only has about 25,000 and reman. trans has about 10,000. About a year ago it started having trouble climbing hills and mileage dropped from 15 to 12 where it has stayed since. Some days it seems to run ok and others not good at all. Sometimes when it's struggling it's like something suddenly kicks in and it takes off like it used to. Also, it will sometimes stall when put in gear -this can happen when it's cold or warmed up. The check engine light seldom comes on and when it has the only thing it has indicated was o2 sensors which didn't help. Here's what has been replaced over the last year or so by three different mechanics with little to no improvement:

air filter
fuel filter
gas cap
mass air flow sensor (twice)
o2 sensors (twice)
ECM (it was a used one but problem remained exactly the same so I'm thinking it wasn't the problem. Faulty logic?)
pcv valve
spark plugs
fuel injectors cleaned but not replaced
catalytic convertor
air intake tube
throttle position sensor
air temp. sensor
coolant temp. sensor
clean throttle body
check intake bolts - they were all tight

The check engine light is not on now but it is definitely not running right which has my mechanic kind of stumped. Any help would be greatly appreciated.