Have a TH350 with a shift kit for the street behind a 350, about a 2400 stall converter. If I leave it in drive it shifts normal. (up shifts and down shifts). If I'm cruising along and drop it into 2nd from drive, give it some gas-like when passing someone -the motor revs up and the trans slowly slips into gear, like when a standard clutch slips. Once it slips into second it stays without anymore slipping until I go back to drive and back into 2nd again. All other shifts seem fine. The trans was fine for about 3 years, this started happening about 2 years ago. Doesn't seem to have gotten any worse. Just annoying when trying to blow the doors (um ah) I mean pass someone carefully.

Any trans experts have an idea what might be the cause of this?

Thanks
REJ