TH350 2nd gear slip?

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. :LOL:

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

Thanks
REJ

Try one thing first and get back to us. Adjust the modulator with a pocket screw driver in until it seats. This will help us determine where you need to go. Even try back the screw out a whole bunch. See if it changes anything. I feel you have a modulator line pressure drop at the time you don't need it. Only a hunch, but worth turning a screw to tell us. We have many techs here that have a variety of experience to head you in the right direction.