1200 Bandit rough running

So the bike has been sounding and feeling a bit 'off' lately. Took it to the mechanics who cleaned and balanced the carbs at the annual service, they also reset the idle speed which was a tiny bit too high.  Trouble is now, the idle speed is very unreliable, and even when warmed up is fluctuating between 1200 and 800 (or zero!).  Plus the bike sounds terrible at anything below around 3,000 revs.  It backfires, clunks (alarmingly from within the engine!) and runs really rough and has little power.  Getting around is Ok - I can just stay in a lower gear and keep the revs up. Problem is in city traffic when it feels like is will stall at any second and it is as if half the cylinders aren't working.  Acceleration ? - fine at higher revs. Almost didnt make it to the top storey of the multi storey car park !  I am going to take it back to the mechanics, but any ideas what the problem might be.  There is no smoke at least....

Mixture problem... sucking air ??? small rubber caps stuffed on carbs where you balance them  or a vacuum hose , or cracked carby rubber mounting flange perhaps . Nothing else changed on the bike prior to this ?( fitted a K & N filter with after market exhaust without a rejet )

Almost as bad is the inconsistency!  Today. fired it up and came into work and it purred like a pussycat!  Before yesterday it had been standing for 4 days, but that is the only difference.  It makes mornings exciting - am I getting the 1200 beast or a 600 cc 240kg lump!

Hi,
At a rough guess,  
Low speed/idle jet problem - crap clogging it up,
air leak on vacc plugs.
The backfire and clunking. My GSX1100G started that, straight to shop, cam chain tensioner was RS and chain was stretched. Check & fix cheapest first.

Cheers GSX.  Strange thing is, the problem has not returned since Monday. Booked in next week for tensioner chain check.  If I had a garage I may try fixing these things myself. Not the same league at all, but I had similar problems on a CB200 in 1981!  Idle jet and cam tensioner issues, easy engine removal and Mum let me put newspaper over the carpet in the front room so I could work on it inside.

Any word on the cause yet ?

I'm cursed!  For the last four weeks it has been fine.  The mechanic has told me to head straight there as soon as it happens again.

From a bank balance point of view, I think you mean Blessed. May have been a bit of crappy fuel and mine hate any of that "special or premium" stuff. Topped up with it once, didn't think I would make the 6kms to home. Need to alter timing if running on e10 as well, so plan old unleaded for me.
Thanks for the update, have a good break, .