Since all they did was change the fluid/filter then you likely have a fluid quantity issue. The problem you are discribing in the context of your description is a torque converter that is lacking enough fluid to operate properly. On the 727 the fluid passage is blocked when the transmission is in park...to fill the torque converter to proper operating levels when the case has low quantity you need to put the transmission in neutral for a brief warm up period. Checking the level on a 727 needs to be done with the transmission in neutral and the vehicle idling. Make sure and fill the transmission to proper level (not over or under). With a torque converter that is slipping due to lack of fluid it doesn't take long to burn up all of the friction plate material.