Can you tell if the Valve has a Closed Center? it kinda fits the problem your having.
I think it has an open center, because it runs fine while the valve is in the Nuetral poistion. Works ok going forward just stalling when I go back.. The only thing I've changed on the system is the motor. same pump same valve.. the guy I bought it from said it had a 12HP B&S. I debating about hooking the cyl up to my neighbours Backhoe He used quick Diso's and runs his spitter off it. It would at least take something out of the equation