Hi, I'm new to this forum and this is my first post.

I have a problem with my Nexus gearhub SG-7C18.
First of all I had a problem with it being stiff and I haven't had it cleaned in 3 years, so I decided to do a complete service on it by myself because no mechanic here will touch it.
I disassembled the whole hub including the though return spring on the axle unit. Cleaned everything very well with chemical benzine. It was veeery dirty.
Assembled it with no problems at all thanks to a very good manual, well the return spring was a pain, but I got it right after some time.
I used the recommended Shimano grease on all moving parts.
Now my backwheel is spinning really nice, smooth and alot less friction than before when shifting gears.

But here is my problem, the gears up to 5 are no problem, but 6 and 7 are slipping. It's adjusted as it should be and I can't see any problems with the chain, sprocket or the shifter and wire. So it has to be something in the hub. I can't remember I saw any damaged parts when I examined them thoroughly.

Any idea where in the hub the problem occurs? I guess someone here has some inside technical experiences with these hubs?