I have an 81 300D with a 722.118 transmission that has been rebuilt within the last year. The transmission shifted smoothly in stop and go city driving. The problem arose, and the reason for the rebuild, was that after sustained expressway driving of 20 miles or more, and then a stop for some city driving, the transmission would start flaring during the 2-3rd shift if it was under any load at all (like going up slight incline). Frustratingly, it has started flaring again after expressway driving and it's not even a year after the rebuild. They tried adjusting the vacuum switch under the transmission so that the 1-2 shift was harder and that seems to firm up the 2-3 shift, sometimes. Whatever it is it is definitely still there and directly related to sustained expressway driving (heat? transmission fluid? vacuum?). I don't believe the 722.1 has a K-1 spring to change. Anyone have any ideas or know of a knowledgeable 722.1 transmission mechanic in or near the San Diego area?
I think I need a second opinion.

I bought a '82 SD with a bad trans. It would work ok for 10 minutes or so, then unless the car was not already in 4th gear it would not shift into it.
The explanation is that the clutch packs (this is not something I understand
but have read) leaked internally and the trans fluid was thick enough when cold but viscosity was reduced when warm and the leaks prevented shifting.
I did drive the car for a while like this. Added 3-4 quarts of lucas trans fluid
(which is like 90w trans fluid) and it seemed to go into 4th but still slipped a little, but driveable. I would not recommend the lucas fluid unless you have nothing to lose insofar as a transmission. Hopefully my scenerio is not related to yours, perhaps the problem can be fixed by adjusting the bowden cable or vacuum to the trans. Make sure the adjustment is right. I'm not an expert, so don't assume the worst yet.

where located the bowden cable in the transmision thank: this is sdl turbo dieselP

Either the transmission was not properly rebuilt, or, more likely, you've got vacuum issues. If the trans is supplied with too much vacuum, it's likely to flare on the 2-3. There are procedures to adjust the vacuum to the modulator, but, you'll need to find a competant mechanic who knows about the proper procedures, or, you'll have to learn them yourself.