Did you purchase a new modified oil pump backing plate? Did you install the new gasket for the modifed plate? Were you having problems before the rebuild, slipping, etc. I would say torque converter is a must replacement item when rebuilding atx.
If you did not rebuild the oil pump, this could have been part of your problem in the first place. The fact that a similar problem occurs in all gears, I think rules out the solonoid body.
How did you install the new piston seals? Did you use a seal installer kit? Did you run pressure checks before final assembly?
Bob