The condition you describe can also be caused by being overfull, when foaming occurs - the expressway is the common denominator. A packed trans filter will do similar.
If it hasn't been done recently I would suggest servicing. The filter can be diagnosed by one who has seen the problem; the paper part will be buckled inward at the inlet.
If the symptoms go away, don't worry about it.
BTW there are plenty of good sources for OE tranny parts. If you can't find any email me. They won't come in a kit except for the gasket set. Don't use anything but OE seals and clutches. By OE I mean in MB packaging. We have had deficiencies with all the aftermaket sets.
Bosch Master, ASE Master, L1
33 years MB technician