Tranny problem

I have a 94 T-Bird, 4.6, 110K miles.

Sometimes my car doesn't want to shift out of 1st gear.
Seems to happen more often in the winter when it is cold
out or in the summer when it is hot/humid. Doesn't seem
to happen as much in Spring/Fall.

I will start up the car and immediately it won't shift out of
1st. What I have to do is shift from drive down to 2nd and
then when I get going shift back up into drive and into 3rd.
After I do that just one time the car shifts out of 1st gear
fine until I turn it off. Then it might have that same problem
a few days later or maybe no problem for a few months.

Looking back at my old bills I see that I had the tranny flushed
back at 70K. Doesn't say if they replaced the filter at that time,
but I would figure they had.

I had a Ford dealer check it out with their computer and
it couldn't find anything electrically wrong with the tranny.

Is this problem with it not shifting out of 1st a good sign
that the tranny is going out? I know that 94 T-Birds aren't
known for having great transmissions.

Thanks for any help.

So does anyone have any thoughts on my transmission and
if it not shifting out of 1st gear is a good sign that it is going
to need to be rebuilt in the near future?

Has the car sat up for a while?

This happens sometimes to the seals when they sit for a while!