gear not shifting into 4th on 1991 Mercedes-Benz 300SE

anyone out there have problems with their gears not shifting into 4th.. my car runs smoothly, until it gets to hot outside or Ive ran it more than an hour on a hot day then wont shift into 4th.. it has only 100,000. somone suggested a transmission flush while my dad thinks it could be a sensor on the outside of the transmission.

Asked by for the 1991 Mercedes-Benz 300SE
It may seam like it does not shift into 4th, but this car and many other Mercedes do not start out in 1st. Try this, while going down the freeway, about 65 mph or so, move gear selector into 3rd position, you should feel the engine rev higher and the car will feel like it wants to slow down. If this happens, the car has gone from 4th to 3rd gear. Under no circumstance - DO NOT SHIFT INTO 2ND AT THIS SPEED - YOU WILL RUIN THE TRANSMISSION AND ENGINE!