Trans sticking in 2nd when cold

[irobertson]

Hi folks,
I picked up a lovely 1991 300SE a while back.
It runs very well except for one thing, it is hesitant to shift out of 2nd when cold. If I drive away without warming it up fully, it will stick in second for about a minute until it shifts. Once it shifts once, it's fine after that.
Checked the fluid level and it is just slightly below full.

Thoughts on this.... anyone experience similar symptoms?