weird stumble at 1400-1600rpm

[shake1150]

hi all,

quick question to all the gear-heads out there...

my 2006 R1150R twin spark (45,000 km with full up-to-date service) is running really great... but...

when the bike is cold, it stumbles or hesitates when the bike is idling (or constant throttle) between 1400-1600 rpm.
for example, when I start the bike with the fast idle lever, it will idle steady and strong at around 1400 rpm, but occasionally stumble down to 800 rpm or so for a quick second. even with the fast idle lever turned off, if you hold the throttle at a constant 1400-1600 rpm, you can recreate the stumble. after the bike has been fully warmed up, i cant get it to recreate the stumble. otherwise, the bike runs really well. its never done this before, so I'm not sure what to make of it.

could this be the stick coils? or a fuel issue?

any insight is greatly appreciated.

thanks!
shake