The idling while sitting in traffic for that long will overheat a bike, and probably explain why it happened so much quicker this time.
What confuses me is that it recovers as soon as you let off a little. If it is truly temperature dependent, then it doesn't seem that it would completely recover just by backing off slightly. It seems to also have an rpm component, or maybe it is vibration dependent. Does it happen only in 5th, or if you downshift to 4th is it still sputtering at high rpms?
These are the hardest problems to troubleshoot due to the intermittent nature, and this one is particularly difficult, because it seems that you can only troubleshoot it while going 70 mph at WOT.
Something is breaking down due to temperature and load or rpm or vibration. Figuring out what that is without just changing out parts is the difficult part. It can also get pretty expensive if you don't find it quickly. A buddy with a Rebel to swap parts with would be the best scenario for finding it without replacing good parts, but you don't list your location.
I would start with all electrical connections. A huge percentage of electrical problems are wiring and connection related. Service manuals for EFI systems often claim that 80% of EFI problems are wiring or connector related. Disconnect everything, clean with electrical contact cleaner, be sure connections are tight, or squeeze female connectors to make them tighter, coat with silicon dielectric compound, and reconnect tightly to insure good connections. Look for chafed wires, bare spots, broken wires, or any other bad spots in the wiring harness. Just wiggling, pulling, pushing, tugging, and moving things can sometimes either fix a mysterious gremlin, or make it fail completely so you know what to replace.
If that doesn't do it, hopefully someone has had some similar electrical gremlins who can at least point you toward which parts may be most likely to start with, or maybe have some more tests you can run. With no other info, I guess I would start with the cheapest things first, spark plugs, plug wires, etc.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
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