I don't thing a spring that's too strong will hurt the solenoid,it may not let the valve move or coil bind and not let it move as far as it should.Maybe the trans was not preped right for the valvebody You have.Fluid supply holes to the reverse piston must be drilled in the case to dump the fluid fester,an air bleed hole should be at the top of the piston so You don't have compresed air that won't let the piston release quickly.stronger piston return springs are sometimes used also.I'm just guessing at things that may or may not have something to do with Your trans.Bill.