Gear changing problem

[amb1947]

Help! I am not very technical. I have a 2006 C 220 CDI Coupe, I have just returned from driving 2000 miles in France towing a caravan and used the cruise control all the time, the gearbox was set to 'S'. At the end of the journey noticed that I had lost the ability to kick-down to engage a lower gear for overtaking. I switched to 'manual' mode and was able to change down but when in came to manually changing up the gear indicator on the dash showed 'D' after '4' instead of '5'? The diagnostic light came on to the right of the rev counter. I noticed some light and recent black sooting deposits on the rear bumper which I washed off. The diagnostic light went out the next day. I took it into Mercedes who put it on the diagnostic machine. They said that the stored error codes showed that the Intercooler had reported a problem but that the vehicle was showing no faults now. They carried out a visual inspection and could find no leeks or other issues. I drove 40 miles yesterday everything was okay at the beginning of the journey and then I again lost the ability to kick-down to engage a lower gear for overtaking. I switched to 'manual' mode and was able to change down but when it came to manually changing up the gear indicator on the dash again showed 'D' instead of '5'. There was no black sooting deposit this time and no diagnostic light on. I noticed when driving up a hill that the gearbox did not change down but stayed in the gear it was in. Help please!