It seems the filter and fluid need changing,most of the time,that is all it needs,the fluid has become thick,or the filter is getting restricted,and,or both.

It sounds like there is an internal leak or the fluid level is low.
The fluid should be flushed every 20k miles.
I would have the trani inspected. The dealer can tell you if it will be cheap or not to fix. Go ahead and find out. Don't wait it may make matters worse.