I've noticed some issues with my transmission lately (4L60E with TransGo shift kit & 3 washers). When driving at about 55 mph and pressing the gas pedal a bit, the transmission seems to stutter. (If I stomp the pedal at that speed, it'll shift down just fine & take off.) I've also noticed lately that when cruising at interstate speeds, 70+, it seemed like it was running at slightly higher rpms than normal. The other day I confirmed my suspicions, when it completely went into overdrive while just cruising with the cruise control set. It's not staying completely in 3rd gear, because if I manually shift down, the rpms are a lot higher. Seems like it's somewhere in between? Is this maybe something to do with the stall converter? I really don't understand much about transmissions.

 

Did you have the PCM tuned after installing the high stall converter?
Have you scanned the PCM for codes?

3 washers in the 1-2 accumulator can be hard on the drivetrain and trans parts. Next time you have the pan off, reduce that to at most 2 washers. Several pro builders here recommend no more than 1 washer.

 

From your description, it is not staying in 4th gear (overdrive). The rpms are going up and down slightly due to the torque converter not staying locked up. The stutter you are having might be the torque converter clutch not holding and coming on and off rapidly. Duplicate the condition when it stuttered and then touch the brake pedal to get the torque converter clutch off and then give it throttle just like before when it stuttered. If it does not stutter, then it is the torque converter clutch causing this. I never offer or advise the use of the washers in the 2nd accumulator.