722.6 Transmission changes early

[Donald]

Have a 2003 c200 with 126k kms. Transmission changes up when especially when cold and when you accelerate it realises its in the wrong gear and clunks back. When driving along drops into 5th at 45kmh. Totally different to my SLK230. If you drive it calmly when its warm it seems OK. It becomes problematic starting ans topping and going over speed humps as it is always in a gear to high.
Local Indy has serviced it twice as oil was dirty, changed the plug and orings and no contamination. Reset the adaptive settings
Slight difference but still not acceptable.
He had a second hand valve body and conductor plate which has swapped around to isolate the problem.
Now thinking that it could be:
temperature sensors, but which ones?
MAF?
TCU?
Does TCU replacement need dealer intervention.
Any other suggestions would be welcome, could it be valve or solenoid related.