If you are able to determin its not the clutch then there are a few other things you can check. The bulk head is prone to splitting around the clutch master cylinder. To cure you will need a plate welded over it to secure. The other is checking the gear stick in 5th gear to see if it moves when accelerating and decelerating. If so you you will need to tighten the two main shaft nuts in the gearbox. Hope you get it sorted
