Well, for starters (pun intended), if your bike fires right up w/o the enrichener, then it is getting more than enough fuel through the pilot circuit. Probably too much if it runs smoothly on a cold motor, no "choke" (enrichener).
My initial thoughts...the NBZT needles should NOT need to be shimmed if the mains ate properly sized...the mains are prolly undersized by 1 or 2 steps...the vacuum piston (slide) air bleed hole is too large (although if you are running OEM slide springs, I doubt that is the cause.
Could it be an intermittent vacuum leak? Yes, it could. Check everything with the methods of your choice.
If mine, here is what I'd do:
Check all caps & connections for splits/tears/leaks...
Pull the needle shims...
Install 150 or 152 Keihin mains
