I would try a trans. oil change to a synthetic while waiting for a rebuild. A thinner gear lube will also help, and I believe the synthetic is a thinner critter, but protects better. Royal Purple is said to be good. They also have some additives that should be considered along with the synthetic gear lube for high mileage manual transmissions. If and when you do a rebuild, consider looking into some mods that improve the performance of the trans.
I forgot to add that a dragging clutch will cause your problem. Check to see that the clutch is fully disengaging when hot. If not, adjust the clutch linkage. Also, check the condition of the clutch, pressure plate, and flywheel assembly by using a feeler gage to mesure distance between pressure plate and clutch, and clutch and flywheel, with clutch peddel depressed. Rotate the whole assembly 1/4 of a turn for each messurement. If messurements are not uniform, your clutch and or pressure plate are warped and a new clutch, and pressure plate (w/through out bearing) are required. The fly wheel will also need machined flat.