You may have a bit of miss adjustment between the throttle and transmission pump levers/linkage. The engine should slightly increase RPM (from about 650/700 rpm to about 850 rpm) before the transmission tries to move, and then they should be working pretty much together. I am not speaking about this particular model and serial number, (so the factory specs for RPM may be higher or just different) but for hydrostatic transmission powered equipment in general.
I don't think I can recall how Clark put the linkage together for a Hydrostatic.