That is a very strange issue.
Sounds like a problem either in the valve body or the governor. Either the valve body section that is controlled by the kickdown cable is incorrectly going to a setting that looks like full throttle causing the downshift, or the governor faulty and is simulating a lower road speed than the car is actually doing, causing it to call for a lower gear?
Does the engine actually overrev, like beyond the redline? Or does it still choose a gear that is lower than requested, but within the engine's speed parameters? If it still keeps revs below ~4500 then that would suggest the governor is OK perhaps, since I don't think this trans has any reference of crankshaft speed to control revs otherwise. But if it's shooting the revs to 5000 or more then maybe a governor fault is a possibility.
The ZF trans sometimes can have a governor problem that results in delayed shifts out of first gear, usually caused by buildup of clutch material in the governor from a problem elsewhere in the trans. But I don't think I have ever heard of that issue causing problems at highway speed. Normally with this issue, once it shifts out of first on the first upshift after a cold start, things work as they should.
I'm trying to think what else could cause this to be related to higher road speeds. Could the pickup screen inside the transmission pan be loose and allowing the pump to suck in some air when spinning the pump at high speeds for extended time (ie highway travel)? Maybe combined with slightly lower than correct fluid level? Aerated fluid could cause all sorts of misbehavior in the valve body I would imagine. Or if the pickup screen is clogged and restricted perhaps similar issues could surface.
Maybe next time you travel on the highway you could pull over after approaching the speed where it acts up, and pull out the dipstick and see if the fluid looks foamy or has air bubbles in it? And also of course, have you confirmed the fluid is full to the correct level and looks/smells normal? No water milkshake, not extremely dark, etc?
Beyond those checks I think my first steps would be to do a complete normal transmission service, changing the fluid and dropping the pan to replace the pickup screen and look for problems for how the old one was installed/broken O-ring or clogged screen, ensure new screen is installed successfully, and inspect inside pan and on the magnets for foreign material or other signs of trouble before cleaning it up well and reassembling. Many automatic trans misbehaviors can be solved just with a fluid refresh if the fluid has not been serviced in a long time. At least that has been my own experience. You often never know when it was last done, whether the correct fluid was used, etc. The transfusion method is my favorite for getting all the old stuff out but even just draining and filling can help.
I very much doubt it has an AW71 as those to my knowledge were never used behind the diesel engines. Diesels in 7/9 series only had ZF 4HP trans. If you want to confirm, look at the transmission pan at the bottom of the trans. If it has a pan shape that tapers narrower towards the rear and is bolted to the trans with 6 securing clamps and a rubber gasket, then it is the ZF which is what you should expect. If instead you see a square shaped pan that is installed with bolts going through a flange around the perimeter and a cork gasket, then that would be an AW trans and we will be very interested in how it is mated up to the engine.
