If there is ATF in the coolant, there is a chance that there is coolant in the ATF. This will do a number on your tranny is there is enough of it. I would drain the tranny and take a look. Better yet, after you change out the radiator, drain the tranny and treat it to a new filter & gasket.

I would also check the head gasket out, maybe a compression test/leakdown.

Just my humble opinions. Good luck!

__________________
Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)