94 740 il tranny problem

About a week ago, after the car reached normal operating temp (running about 30 mins ) I put it in reverse with no response from the trans. All other positions shifted fine. While driving it shifted fine. I revved the engine and it kind of slammed into gear. After the car cooled down it shifts normally into reverse. Then again after it warmed up the trans would not engage in reverse unless i revved the enging. Fearing damage to the trans i stopped doing that. If anyone has experienced this I would appreciate your comments.

Thanks !!!