I pulled the Cougar out and ran it to the transmission shop. They had it16 days. No quote, no calls. promises made and broken. I got it back with a new torque converter, and a rebuilt transmission in it. The bill was $1500. I took it back for a 2 week follow-up for the warranty. He checked the fluid and said I was good to go. I had a TH350 rebuilt there 13 years ago. New owners. I am not going back ever. I will steer anyone away from this shop. They may have put all their best efforts into a meticulously rebuild transmission... but their practices every step of the way lead me to believe that they probably didn't.
Probably took that long to find parts for it. I had to wait awhile on a 1957 T-Bird Tranny.
Band adjustment will not help unless the lock nut came loose and the adjusting screw backs off. Dextron has a high coefficient of friction which results in firmer shifts.
Type F has a lower C of F, tends to varnish(oxidize) and will cause band failure,especially front.
You didn't mention slipping in 3rd so I guess you meant rear band failure.
More likely hard and leaky O ring seal in the rear servo or the case could have been cracked
in the center where the Rear Servo bolts.
The Torque Converter may have had too much end play or maybe the hub was badly
scored. You never know unless you ask for the old parts,at least see them.! A car that old I doubt if they keep bad parts lying around to fool you with. If the band was damaged that would account for heavy debris in your transmission due to scoring the planetary assy and needing a torque converter.The early
ones you could take them apart and clean. In the future ask them to save your old parts.
Finding an honest technician is like rolling the dice I look at the shop,is it clean and
other than a dealer they all use used parts. Which is ok But you may be getting the wrong ones.
Like that rear servo,they come in all different piston sizes ect.
http://www.supermotors.net/registry/media/855935 my wife's and I won't let her work on it.