Transmission shifting into neutral while driving?

Changed extension housing on transmission and put it back together. When I took it for the first drive it was doing fine but then slipped out of gear around 35-40 miles an hour felt like neutral. Drives fine I'm first and second so remove the valve body Servo spring seems to be okay and snap ring is intact. Cleaned the valve body and had a couple of valves that were dirty and didn't move great, but I did get everything moving freely but still having the same issue. However, if I keep my foot on the gas accelerating it will go through all the gears fine until I let go of the gas pedal at which time it flipped back into neutral until I come back to a stop, and it will take off in first again.