Probably the clutch pack is worn, or else internal seals. The only thing I know of to try is to have the fluid and filter changed, and then add a container of Lucas transmission additive to see if that helps. But it's probably time for an overhaul.
A word of caution. My mechanic tells me that, if the transmission fluid hasn't been changed for a while, to drop the pan and do a filter change first. After a thousand miles or so, then do the complete fluid change. His experience is that doing a change can often cause the transmission to go out. I had never heard this before, but it sure explains some of the transmissions I have replaced in the past.