First thing I'd do is check for leaks in the boots between the throttle bodies and and the cylinder head. They are getting old. Spray some starter fluid/butane/propane around that area when it's idling and see if the idle increases. If that's the cause then what happens is that the upper RPMs generate enough vacuum for those leaks to seal temporarily which is why it will run better at high RPMs.
For 2 valve bikes the part number is 11611460408 and they cost about $30 each. (If you remove the fuel injectors on a two valve K bike then be sure to clean around their bases prior to pulling them from the cylinder head because dirt and gravel builds up there which will fall into the cylinder head if you're not careful.)
For 4 valve bikes the part number is 11611461621 and they cost about $25 each. The O-rings for them are part 11611465169 and should also be replaced. Mystic Red did a write-up on replacing them on a 4 valve K
here.While you have the throttle bodies off its also a good idea to
replace the fuel lines.
