Fluid level?
Has the tranny been serviced ever? fluid and filters?
The fluid breaks down with subjection to heat, and may be corrupted by water ( condensation- short starts) and by clutch materials
what harms trannys is the seals becoming inflexible and fluid not being directed in the correct direction at the correct pressure.
a stop-gap is the tranny additives you'll find at the auto parts store- most of these have a chemical which will soften hard seals and temporarily provide better sealing through the tranny.
if this works, then you know that the tranny will need a rebuild sometime soon.
so you could try a $10 bottle of goop, if it has an effect, then you know that it isn;t in your head and action will be required at some point...
and you have the luxury to pick the time and place of repair, rather than 1/2 way to somewhere else.
Mike