Hi, could someone help me diagnose a pretty strange unpredictable down-shifting problem.
This usually happens only sometimes, mostly when car is wormed-up.
When driving on highway in 4th the car would just downshift into third and would no longer want to go into 4th.
Then, just recently it even went into 2nd and would no longer go to 3rd; putting it into N while driving and back to D would make it go into 3rd again, but would soon again downshift to 2nd.....
Other days it would just drive and shift perfectly....

I did already try following:
Disconnected kick-down switch
Change fluid and filter
Make sure the fluid level is correct

I was told recently that there might be somewhere ion the side a transmission speed sensor and that bad contact there could cause problems like this....sorry, but really do not understand how this transmission is supposed to work and where to look for this sensor, if there is one?

The kickdown solenoid is on the rear of the transmission. It causes the transmission to drop down one gear when you press the accelerator to the floor. If it's stuck, the problem may be the switch, which is located under the accelerator pedal. You probably have to remove the carpet to get to it.

A quick, painless way to test would be to pull out the Klima relay, which is under the hood. Power for the kickdown system is drawn from the Klima, so if the relay is pulled, the problem would go away.