So I just bought a '99 with the 4R70W with 84k miles and it seems to have a little bump or jerk on the 1-2 shift, almost like a shift kit. Its an intermittent thing and I can only get it to do it at light throttle and when it shifts out of first below about 2000 rpm. Even then, it doesn't do it all the time.

When it does it, everything seems like it will shift smoothly and normal and then about 1/2 way through the shift (timewise) it just goes bump and there is second gear. If I have my foot in it very far and it shifts above that rpm, its smooth just like the rest of the gears.

If it were a broken spring in the accumulator, I assume it would happen all the time. Anyone have any thoughts?? The rest of the gears seem fine, and if anything 3rd and 4th are smoother than I would expect. I barely feel them shift.

 

I would go ahead and check the 1-2 accumulator and spring. Sometimes only a couple of coils break off the spring, causing erratic symptoms.