about the transmission on 1994 Toyota Corolla

My car seemed to be fine until recentley.
You can be driving along and all at once it acts like the transmission is slipping. The overdrive button does not work due to my car being hit in the rear. Sometimes you canhold it in for a while and it will catch and work. I also had a CD/radio put in and someone said if the hot wire was not hooked up right that could cause it. I have had the transmission checked at the trans. shop they could not fine

Asked by for the 1994 Toyota Corolla
It may just be that your O/D (overdrive) button is going bad. Also, when was the lst time you had your trans fluid and filter changed and the trans flushed?