is my transmission shot even though sometimes the car will engage in gear?

What is your question?

happened on freeway,6 days after purchasing from private party. After towing home, have occasionally been able to engage Reverse than shifted to drive.We drove it 3/4 way around block, then coasted it home. We want to make sure #1: can a solenoid or a malfunction computer code have caused this.#2:could it just need a complete tranny flush and filter(change)#3: could it be a relay of some type?