Odd transmission quirk

C6 trans with Transgo shift kit and I installed an R-code servo awhile back. While driving at a light cruise, maybe 20 mph the transmission would start to brake and slow the car down to a complete stop, then release.
My foot isnt on the brake pedal and linelock works as it should. It did it twice when the trans was cold and never happened again. It wouldnt lock the transmission and skid the wheels, but more like a light brake pedal application. Any ideas?

The shifter is a B&M Pro stick. The shifter left in D was shifting very erratic so I discovered the modulator line had popped off. When the problem first occurred I was using the shifter manually trying to figure out what the heck was wrong with the trans. After hooking the line back up it was back to normal but that doesnt account for the trans slowly locking up would it? My first thought was the cable had slipped or backed off but everything is tight and secure. The only issue I've had with the shifter is the trigger /lever hole worn oblong and unable to upshift into 3rd and the park detent is a tad off being a universal shifter.

Randy

The servo is a Superior brand and I didn't have to machine anything. I installed it and used low air pressure to check if it's working the way it should. As far as the valve body goes, it could be something internal. I put the shift kit in 2-3 years ago along with the servo and it's been working fine until last week. I'm going to be pulling the engine/ trans soon and see if I can see anything obvious. Is there anyway to bench test a a valvebody?

The cable pin is free in 1, 2 , D, N, R. The only real issue is park. I think the detent in a gm trans from reverse to park has a longer throw than a Ford and from what Iv'e heard you can mess up the cable or the valvebody by forcing it into the park shifter detent. I put in a stop bolt to prevent over extending the cable. I'm going to try road testing and see how it goes.

Randy