It is a 1994 F150 5.0 I bought this nice truck with bad motor and then put a fresh HO in it. When I got it in a noticed a slight imbalance "shudder" when I revved it. No biggie, I then noticed it was shuddering at 45mph for a few sec, i put new u joints in to no avail I realized it was TCC.

For a quick fix I put Dr. Trannys instant shudder fixx-- Well, it worked, quit shuddering. But 2 days later it started to shudder at idle and progressively got worse even with a complete flush-n-fill. It feels like i have the wrong balancer on it( now parked for repairs). I know it is from the converter because soon as i put the additive in it changed almost over night.
Absolutely nothing loose!
Can it be a broken fin in the converter-- any ideas, helpful. I do plan on a new Tranny in the future- just haven't been able to hear a converter problem at idle.

Truck runs fine no misses or issues like that. No codes (at least with jumper wire)or blinking OD light-- shifts hard 1-2 during hard acceleration.

 

Just in case someone else has similar problem--
It was Torque converter shudder. I put Dr Tranny's fixx in that solved it-- I then changed fluid all together that fixed that.

Main problem was Harmonic Balance has spun and I could tell it was vibrating until it started to to come off. I was almost going to buy a new converter I then noticed the balancer was shot. Put a new one on and a new gasket. Everything GREAT!!