I've seen similar symptoms from two other causes as well.

Start with adjusting the bowden cable - make it a little bit longer. The bowden cable tells the transmission what the throttle position is - closed, wide open, somewhere in between. The transmission in turn uses this input to decide when to upshift - 2K RPM, redline, somewhere in between. There is an adjuster for the bowden cable, try turning it in - slackening the cable - to see if it has an effect. On my old 123 car the bowden cable liked to tighten up over time, eventually the car would hang in third gear.

The other cause I've seen is really wacky, but it happened in one of my 124 cars. What is the condition of your battery? I had a nearly dead Optima at one time - it wouldn't take or hold a charge. The alternator/regulator turned the voltage waaaayyy up in an attempt to charge it, and all kinds of crazy stuff happened. One thing was the transmission hung in third gear - the kickdown relay was getting triggered. (It also caused the a/c compressor to run in EC mode.) Replacing the battery cured everything. Strange but true.

- JimY