Possible Transmission Slip

Well I cant figure this problem out on my own. The car is running fine and I can get it to go as fast as I want with no problems. The issue I have though is that at speeds above 45 or 50 when I maintain speed the RPMs will sometimes drop slightly causing the car to shake. There are no problems until I hit this speed. If i let go of the gas and hit it again the problem goes away or if i accelerate and give it gas it also quits shaking. It does this continually as I drive at or above these ranges. Any idea what it could be?

sounds like a torque converter lock-up solenoid going bad, as that is the threshold speed at which it engages.

sounds like a converter locking,,,un-locking,,,,,can be caused by tcc valve not working correctly....valve has issues with side loading causing it not to seat in lock position... Sonnax after market parts resolve this problem,,,but this repair is not a back yard job.
You may want to talk to a trans shop,,,requires valve body removal,,some shops don't want to deal with this type of repair liability