I have a 97 ranger 6 cyl. auto. 135,000 miles truck runs great has developed a problem shifting,in the am. get in crank up back up,no problem,shift to drive may go may not,let sit running for 2 min.or so then it goes and no more problems, only first trip of day.even in summer.thinking of changing filter and fluid,fluid is clean and red.any advise?