You really need to find a way to post a video or something of what's going on. These symptoms don't really make a lot of sense.
A manual transmission has no slip. Your gears are fixed and you will be at a fixed rpm any time you are going a certain speed in a certain gear with a certain tire size. This can not vary and if it does, the only thing that can cause that is the clutch.
Throw it in 6th gear at low speed and floor it and take a video. Otherwise we are just guessing. The descriptions don't really make sense but theory says the only thing that can be wrong based on what you are describing as slippage would be a bad slipping clutch.
A manual transmission has no slip. Your gears are fixed and you will be at a fixed rpm any time you are going a certain speed in a certain gear with a certain tire size. This can not vary and if it does, the only thing that can cause that is the clutch.
Throw it in 6th gear at low speed and floor it and take a video. Otherwise we are just guessing. The descriptions don't really make sense but theory says the only thing that can be wrong based on what you are describing as slippage would be a bad slipping clutch.