Hi Ray
I have not built your particular locomotive, but the problem sounds to me like it may be due to unequal valve movements forward vs reverse. Is your reverse stand in the right position? If you use a dial indicator to measure the valve movements all the way forward then all the way back, are these movements the same distance each way when reverser is in mid gear?
Might also be due to your valves' position on the spindles, or possibly your valve port positions are a bit off. Either way the valve adjustment can probably adjust the working to give more equal events or at least faster forward speed.