Going to check out a very clean C70 this week. Seller says that the transmission works fine until it warms up. Then it slips while slowing down. Any ideas?

 

Since its heat related, I'd look at two things - first check the fluid level and color and possibly consider dropping the pan to look for debris (ie from damage to the internal clutches etc, then measure impedence of the solenoids. Slipping can be caused by low fluid levels, weak solenoids (solenoids are essentially switches operated by electromagnets. A dirty or weak solenoid may restrict the flow of fluid) or worn components like the bands or clutches. Clean fluid at the correct level can be done with a drain/fill DYIers can do, solenoids can be replaced with the transmission in the car but the worn components point to a rebuild or replacement tranny. Best to price your purchase on the assumption you need to swap in a replacement tranny and go from there.