Well it all depends on what you're expecting from your 6th gear. 1st thru 5th are the racing gears but 6th is the cruise mode gear is all, never intended to pull torque or for passing slow moving vehicles. I'm thinking you aren't using the Harley 6th gear as the MoCo intended or I'm not getting your message clear.

 

You kinda did the hard stuff first, have you checked for a loose drive belt? Any chance you are having an occasional engine miss that gives the impression that something in the drive just slipped? How many miles on your bike and how many miles on the spark plugs? Have you checked the ignition wires for chafing? Is it due a fuel filter? Is the air cleaner in good shape or is it dirty? Are your battery cables clean and tight? You might be due some fuel injector cleaner. You only have a 5 or six gallon tank, don't pour in the whole bottle. Typically it calls for one ounce per gallon BUT read the label on whatever you buy and go by that guidance.