Transmission slipping

Probably...

Is it doing it while using the cruise control??

try locking out the overdrive when you are below 55 mph. there is a service bulliten on this problem. the tranny overheats due to low fluid flow while in overdrive at this speed. the official fix is to replace the spacer behind the valve body i believe. the tranny will also slip at this speed in od when it is really cold. one of the most common causes for this though is improper transmission fluid. dodge also has a cold temperature bypass kit that helps solve the cold issue. have you had the fluid changed recently? michawel

Perhaps a low $ band adjustment could fix it (guessing). If it's not an expensive problem then it will quickly become one if you let it continue slipping.

I had a the same thing going on with my '99 1500 4x4 at about 75k. The torque converter went out, and the metal shavings blew out almost every internal seal and valve in the damn thing. If it wasnt under warrenty, the tranny alone would have been almost $900.