Would the problem be as simple as binding brakes? Happened to me after the bike was laid up for winter, Back brake was dragging and when I kept the speed at around 75 it would gradually drop. Sounded fine (engine) but the local bike expert was suspicious when the bike was hard to push, he took it for a ride and came back and the back disk was so hot you could fry eggs on it!! Stripped & cleaned and no hassle since.
What about fuel mileage SAGS, are you getting more mpg than usual? Are the plugs too white or clean? If so on either point the bike may be running too lean and this usually becomes more noticeable as a lack of power in the high gears.
As far as the brakes go I always change pads every year,worn or not,and if their was any drag I would be sure to investigate.And after rounding out 2 hanger pins,because they were never greased when installed.One on each of the front calipers.Boy you drill careful on $400.00 parts.Lightly grease the threads on the pin and locking top plug,I don't white knuckle them tight either.
You have a good point on the lean buisness,am going to put her down for a speedo fix and will check plugs then.Second speedo cable I have been thru,any ideas on that.Think the plastic part is gone again in the front wheel too.