Hi, my gearbox still going strong so glad I kept at it. Can t say for sure if it would fix your issue but I would say I would definitely drop[ the valve body and have it reconditioned if I were looking to drain the fluid, change the filter - only do it once then and it is not a big job if you are handy.
Also maybe check the (3) electrical harnesses where they connect into the transmission while you are under the car, probably full of oil but doesn't normally seem to affect the running as I have had three of these and they all had the same issue.
Good luck