My 1998 plymouth grand voyager has 80,000 miles on it. Lately, while driving at speeds over 40-45 mph it appears to be changing gears or can't seem to find the correct gear. When it does this there is no change in rpm. runs ok around town. if I ease up on the gas pedal at higher speeds and resume pedal pressure I feel and hear it trying to shift or ?. Any ideas? thanks.