If your Sachs Pentasport blocks extreme gears (you can switch only between 2 to 4, gear 1 and 5 can't be chosen because you are afraid of braking the lever) then:
1. Make sure that you've disassembled, cleaned with white spirit, lubricated and assembled gear correctly.
2. Make sure that you are adjusting the lever correctly (starting from gear 4 which means all cables are in their "longest" position and than adjusting length).
3. Check if you've placed sun gear correctly, in my case it was the root of all evil. Befor mounting a wheel to bicycle it seemed to work but after riding for a while gearhub resisted against switching gears 1 and 5, I didn't want to break plastic lever so I've started to investigae what might be the reason. Sun gear can be easily flipped (two side are different but it seems to make no difference but it does make a huge difference).
The part 25a on this drawing is the mentioned gear which can cause problems if flipped.