I also had an 82-83 XT550 only 3 years old at the time(the bike, not me)which ran fine up to about 120 KPH, then lost power down to 100 KPH, then accelerated back up to 120 and so on, all at full throttle.
Turns out it was a intermittently faulty timing advance circuit in the CDI. What pointed me in that direction(after trying everything else) is that I was told at the time that the CDI's are troublesome.
I know these symptoms are different than yours, but this info may be of use.
Bob
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Remember that Assumption is the mother of all f**k-ups.
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