Hey Stefan... maybe your problems is known by the dreaded three words problem : 'Picco XP Curse'.

Seriously: Disassemble the carb and inspect the fuel intake and the spraybar along the low-end needle, your problems should be there, maybe some dirt gone there and causes problems. If this doesn'ts solve the problem, swap the carb for a well known NR carb and your problems will be history (well... maybe Reto drop by and give you an answer).