Big Phatso

If everything works out I will be purchasing a 2000 Z28 with A4. Most of the issues with the car are menial and easily fixed. The main issue with the car is that the transmission in drive slips out of 3rd sometimes, sometimes it gets into 3rd but then slips around and may go into 4th or just goes back to second. The current owner states that he was told it could be a solenoid. My question is should I attempt to mess with the car or is this a red flag and should I run? And if I do mess with it, what am I looking at to get the tranny working right? Thank you for your time.