washer turns slow but wont agitate spindle good

Washer works in all sections except when it spins to wash cloths, the motor turns slow and i think it keeps it from agitating.Moved tranny by hand -moves freely both directions.Its not stripped agitator.took off belt while spin cycle starts motors humms and moves slow.Replaced motor and tranny and seloinoid,very puzzled.It works great in all othe selections and spin cycle turns in reverse:confused: