Are the plug wires original?
Is the tranny fluid original?
Misfires certainly can occur only at low RPM under load. A mixture problem would aggrevate this.
It seems likely that the torque converter lockup point is hunting around. This happens in a narrowing speed band as power is increased. Engagement may be faltering if the fluid is old or the PCM may be getting fluxuating data from the sensors about speed and engine load. Cleaning the MAF probably increased the apparent engine load and moved the engagement point up some.
