If the revs climb suddenly in a higher gear, where you cannot be breaking the wheels loose, and then snap back down when you let off the accelerator, that's a slipping clutch. This can happen because it's worn, the cable is tight or binding, the linkage is binding and will not release completely or because the rear seal is leaky and there's oil on the clutch plate.
You are correct that a fluid change will do nothing for this. A manual is either in neutral or it doesn't slip, even a little, without breaking teeth and rending metal.
Remove the battery so that you can get to the clutch lever on the trans and make sure that it moves freely past the rest position. You'll probably have to slip off the cable end to do this. Make sure that the cable is not taut when it's at rest. If everything is loose and it still slips, it's probably the clutch itself. If it's the seal, that's still located behind the clutch.
John
You are correct that a fluid change will do nothing for this. A manual is either in neutral or it doesn't slip, even a little, without breaking teeth and rending metal.
Remove the battery so that you can get to the clutch lever on the trans and make sure that it moves freely past the rest position. You'll probably have to slip off the cable end to do this. Make sure that the cable is not taut when it's at rest. If everything is loose and it still slips, it's probably the clutch itself. If it's the seal, that's still located behind the clutch.
John