With the list of bikes you've had, your age, and 35 + years riding experience you've probably looked at the obvious .....the external linkage. But if not:
The shift lever, (more accurately the internal shift quadrant), must freely return to it's natural "home base position" I'll call it, in order for the tranny to shift right. You must determine absolutely if thats happening. I suggest you get the bike uprite and the rear wheel off the ground, centerstand works great, so you can sit down on the ground and watch the shift shaft as you work the lever while rocking the rear wheel back and forth. (Rocking wheel is neccessary to facilitate shifting because the engines NOT running and your ignoring clutch use). Focus on the movement of the stubby lever on the shift shaft where it enters the engine case, not the shift lever itself. If thats tough for you, then take the external shift linkage stuff out of the equation completely.....by removing the stubby linkarm from the shift shaft and remove shift lever from it's pivot pin by removing the retaining circlip. Slip the stub lever back onto the shift lever in a position you can work it by hand. If shift shaft doesn't work freeely and snap back to "home-base position" as it should somethings up inside.
If no binding was ever found .... check that the shaft has almost no "in-and-out" play. Well less than 1/8" is proper and while down there check that the circlip on the shaft where it enters the engine is seated in it's groove.
Have you dropped the bike on the shifter side recently? How do you eventually get it to downshift, ( like after it gets stuck in 6th).
dave
