300TD question

[rjm100]

Hi everyone, I'm new to this forum as well as to my 1985 300TD, something happened yesterday as I was driving along a relatively flat highway. Everything seemed fine for the first half hour of my drive, I was going about 100kph(60mph)but then after accelerating up a small incline (very small) and leveling out again it seemed like the car all of a sudden lost some power, I found with the pedal pushed all the way to the floor I could go no faster than 80kph (50mph) and then the for the next few minutes the car would slowly decelerate and eventually I could not get going any faster than 60km (35mph) as all this was going on I was listing to the engine and I could hear the turbo boosting and the rpm increasing at times but it was not pushing the car. It felt like the transmission was not working. I pulled off the highway and let it cool down for a bit and took the backroads home. It drove fine going on the slower city roads, still lots of power and turbo was working, the only other abnormal thing was the few second delay from when you step on the gas from stand still seemed to be a longer delay than normal. Transmission fluid level was ok. Is my transmission going? Is this a common problem? Thanks -Robby