I have a ML 270CDI, 2002, 94000KM and a had exacly this problem.
The enter in service mode (2 gear) may be related with a weak batery.
The problem with 3-rd and 4-rd gear when translate is a computer of transmision problem.

5 day a go my car was fixt and naw it is working very nice.

If you need more details please ask.

Radu Bercan