If the RPMs change but there is no corresponding change in speed, suggest you've got a transmission issue. First and easiest thing is to check the fluid level. Also, if the fluid is not a bright cherry red color, consider having the tranmission serviced - complete fluid change and filter.
A tranmission flush is an option - varied opinions on doing this. Mine is to avoid them - have heard too many horror stories about the transmission failing soon afterward - perhaps the flush is too aggressive?
