Subaru: transmission slips when cold outside

my 92 legacy had been driving fine. I parked it overnight, and the
temperature dropped down to about 32 degrees that evening. When I
started it in the morning, I put it in drive, and I felt the
transmission slipping. The transmission felt as if it was in neutral.
When I put the transmission in (3), it would not slip. it actually
seemed to run fine. But when I would return it to drive, it woud slip.
So I drove it in (3). When the engine temperature increased to the
normal range, I put it back in (D). The transmission stopped slipping
and ran fine all day. After sitting for another cold evening, the
problem has happened again. Any ideas would be appreciated.

Note: The ATF fluid level is normal (full). The fluid is clean, and
does not smell burnt. Thanks again.

Thanks for the input. I will flush the and fill the tranny with new
ATF.
Now the car is doing something new. When started cold, it idles at
around 850 rpm, then when put into gear, it stalls out. If let to idle
until engine is warm, it runs fine. once again, any help would be
apreciated.