My combine hydrostatic drive will not operate in 3rd gear without the hydraulic charge light and buzzer sounding, will move slowly in 2nd gear under load. In neutral the pressure is 250, but drops to 60 when under load. Have checked filters, drained and looked for shrapnel, all clean and no leaks or fluid loss. checked pumps and valves all seem to be ok. has been at the JD dealership since Wed., and can't find the problem. Don't want to start randomly replacing pumps, too expensive. Would welcome any suggestions.