Hellloooo and seasons greetings;

I need some help:

Car: 300e 4matic, 1990
Problem: transmission is hesitant, even with some very noticeable bucking on the first upshift (2nd to 3rd?). It does shift up correctly (when you insist), but reluctantly.
It's not a really a big deal because once at normal temp, all is normal.

FYI notes: trans fluid is Mobil 1 synthetic, with new filter as of last summer. Seems to be at correct level. However it was verrrry cold here the other day (-25 c) in Toronto, I pulled the trans dipstick and the fluid was nearly frozen it was so thick!

I should say that while I only ASSUME it's the tranny, you can sort of sense it by the behaviour.

Any thoughts? Again Season's best to all!