Still pretty stingy with information.
Have you recently done anything to/with your FJR?
Does it turn OFF/ON only once or can it do it many times? Are you unable to ride 65-80 because it won't run in that speed band? I won't ask if it runs reliably faster than 80 MPH 'cause I don't want to be part of you getting a big speeding ticket.
Is it related to a RPM band regardless of gear?
Does it happen only in 5th gear?
Is there anything different about the road or your location in the afternoon vs in the morning? Even something like you get on the highway first thing in the morning and get off shortly, but in the evening you have driven quite a ways before you get on the highway (or some scenario like this)? Is there a change if road surface in the area where you get the drop-out?
Typically, if it is fuel related power fades in and out and doesn't go ON/OFF/ON suddenly. Typically, if it is electrical related it DOES go ON/OFF/ON.
There is nothing that happens only at that speed which would trigger the drop-out, the problem has to be there but not expressed in other riding conditions.
One thing in the back of my mind is the TPS, it can be in a condition where it will only fail when hot, but it will only fail in a narrow RPM band. Do you know if the TPS has been changed to the new design TPS? If you bought your FJR used you can call any Yamaha service desk, give them you VIN and they can tell you what has been done to your bike. The thing that bothers me is if it is the TPS it would run bad in every gear, at any gear specific speed at one fixed RPM range.