insufficient engine output
hill
acceleration demand (from driver)
headwind
bad thermostat
electrical problem
Your transmission might automatically shift out of overdrive while driving on the interstate because of low transmission fluid. This issue might also occur because of a bad transmission sensor or overdrive sensor.
what would transmission not to shift in to overdrive
the overdrive sencer
My 1984 corvette transmission is not shifting into overdrive. It shifts first thru third fine.
If you are city driving, and stopping at many lights. The tranny might be shifting into overdrive constantly for just a few seconds before you have to stop again, which would cause excessive wear on an expensive transmission. Personally, in this situation I would disengage the overdrive.
If the overdrive switch is turned off then the car will not go into overdrive. As it is not in overdrive, then it will not downshift out of overdrive as it normally would. It also will not shift to 3rd gear above a certain factory preset speed. Turn th overdrive on and see if it operates normally.
If you accelerate and the transmission shifts to a lower gear of the overdrive disengages the RPMs will increase. If the vehicle has a manual transmission a slipping clutch will cause the RPMs to increase if the clutch is defective.
If the O/D OFF light is flashing while you are driving you should take the vehicle in for servicing as soon as you can because you can cause damage to the transmission
If the transmission is healthy and any/all adjustments are done properly there should be no slippage.
Usually, it won't go into gear. If you get it into gear, you could cause serious damage to the transmission.
Linkage adjust
It sounds like your having a transmission problem. There is a lot of things that could cause this in a transmission, including a bad siloniood or it could be a clogged filter.