Papalarge, welcome to this super forum. Also, say hello to the black magic that controls the transmission of your MB
From what you describe, my first feeling is that it's related to either the kickdown switch sticking or its the Bowden cable that got a bit rusty or out of proper adjustment.
First the kickdown switch . It's under the accelerator pedal and you will have to lift up the carpet beneath the pedal to see it. It's a small black plastic cylindrical device. Try pressing it down with your hand. The portion that sticks out of its body should get depressed when you press it and spring back to full height when you remove the pressure. If it's sticking, then it's for sure your culprit. You can try lubricating it with some plastic-safe spray lubricant (I think white lithium is safe for plastic ) . Else, you can replace it.
The only problem is that it's not necessary that if the switch is moving fine, the small solenoid its connected to at the transmission is also working fine. But since that takes longer to diagnose then the Bowden cable, you can leave it out as a problem for now.
The second step, if the kickdown switch is working ok is to check the Bowden cable. This part basically controls the upshift points of the transmission by doing some black magic. You can search for how to adjust and inspect it through the search forum and i am sure you will find a ton of information on it. Here is a video by a fellow forum member kindly put on YouTube that explains what all the mystery is
Mercedes Diesel Transmission Adjustment - YouTube.
My idea is based on a different chasis but I think that the same idea applies to diesel mb transmission of this era.
Good luck!