[v8volvo]

Another question: do you observe that the torque converter lockup function is working normally? Assuming the trans is a ZF, the TC should lock up when the car reaches around 45mph (70 km/h) then should unlock again as you slow down to around that speed or a little less (hysteresis).

If lockup isn't working correctly and/or if you notice this problem is closely related to when the converter is or isn't locked, then that might suggest there is a valve body issue related to the TC lockup pressure circuit that is somehow triggering the kickdown circuit when the TC is commanded to lock up. That could perhaps be from a leaky valve body gasket or something else like it, speculating.

That's the only other idea I can imagine that could create such a clear speed threshold for when the trans screws up vs when it acts normally. It comes to mind because the TC behavior does work exactly in that way. And on some cars with taller geared rear axles like I believe the later Euro market diesels had, the speed threshold might in fact be higher like 100 km/h rather than the lower speed where our older US model cars here do it, with their short geared axles intended for 55mph speed limits that were the rule when the cars were new.