About 2 months ago my 1985 rx7 12a motor automatic transmission 109k miles no mods all stock started vibrating badly, worse between 2k rpm to 3k rpm. Smooths out after 3k but still there. If I let off the gas the vibration stops immediately until the engine tries to push the car. Read thru many threads here and have
replaced cap and rotor spark plugs and wires. (needed it) spark plugs on T1 and L1 were badly fouled with whitish, yellowish crud. T2 and L2 looked pretty good.
Put it on a rack and drive train and ujoints are rock solid.
transmission and motor mounts inspected and are good.
Cannot rock or move the engine while it is off.
removed AC belt.
replaced both front wheel bearings (tired), helped a little bit.
No play in rear wheels.
It is like the engine wants to hurl itself out of the car.
Vibration gets bad enough to loosen the front (T @ L 1) coil wire.
replaced fuel filter.
The vibration shook the 35 year old muffler out of the car.
higher rpms by turning off the overdrive helps but kills the milage.

If it was a v8 I would say the harmonic balancer is warped but I don't know enough about these motors to even know whether they have such an item. The belts seem to walk front to rear while running and vibrating. Any ideas would be helpful.
Thank you

 

Maybe a bad torque convertor?

 

if it was a bad torque convertor would it not have dodgy shifting? Shift points are good.

 

Could still be the torque convertor. When was the last time it was serviced with new trans oil in the convertor?

I'd replace the engine and trans mounts. it's cheap and good insurance. Might even need to take the drive shaft out and inspect the u-joints. They can become seized.