I have a 1998 E350 with 140k on it. It has the 5.4 motor and auto trans. It is starting to slip and stay in first gear till I'm doing about 25-30 mph. It seems to do this more on hot days (90+). It doesn't do it every time but periodically. Is there anything I can do to prolong the life of this trans ? What about getting the trans fluid completely flushed by a shop ?

 

Have the trans fluid completely flushed can't hurt, regardless of what some others may tell you.

Your trans may still die right after the flush, but the flush didn't cause it. Once you wait for problems to appear a flush usually can only delay the inevitable failure, it can't prevent it. Sometimes it doesn't prevent it at all.