This first gear issue is also common among mercedes. This happened to me alot. I eventually figured out that it usually happened after downshifting from 2 to 1. For some reason 1st didn't completely engage all the time. When you down shift try going 2 to 1 to N and then back to 1 - make sure you REALLY stick it in good. I eventually had my tranny replaced at 50K, but that was related to my reverse gear taking a crap.