Bike lugs and dies at ~80mph

[mac]

Preface: Hello all, I am new here, and pretty new to riding and have basically zero experience working on a bike.
I have one of Patrick Lajko's old bikes (the black one, 1993, Phoenix I think was the name he had for it), total miles since he rebuilt it is 10K+.  I've put about 1k miles on it since I bought it and it has run as expected, I've lubed the chain in that period of time but not much else.  I haven't taken it above about 85mph.   All that is background to say the bike was very well maintained before I got it, and I haven't beat it up since.

I have only had one issue with it which has now happened twice.   At freeway speeds (70-80 MPH) I've had the engine lose power and start to pulse.  It feels as if I was rolling on and off the throttle and and each time it rolls farther down, and not as far back up.  Both times I have (obviously) had to get to the shoulder very quickly, and the bike has died completely.    It will then restart after 1 to 3 minutes and drive fine, though admittedly I have driven a little slower afterwards.  In both cases it happened after about 10-15 miles of driving, and then road fine for 45+ after.

[The Buddha]

Fuel starvation - may be the gas cap vent needs to be cleaned ? You have a tank bag ? If so, remove that first.
cool.
buddha.

[Janx101]

Fuel line pinched? Tank petcock screw completely vertical? ...

Only at those speeds? .. or also slower road speed/lower gear but same revs? ..

Sure sounds like a fuel starveout from something?!

[bmf]

Nice to see one of Pats bikes back on the forum. +1 for fuel starvation, also check for correctly routed breather hosts on the carb. Incorrectly routed hose can lead to wierd behaviour at highway speeds - a "breeze up the kilt " if you will.
Also try to switch to prime when that happens, would identify any frame petcock issues if it goes way.

[mac]

I put 60 miles on it today, took it to 80 no issues what so ever (and yes, only at those speeds was it happening)

I took the shotgun approach and just did both suggestions above (carb and vent) so I can't say what helped, but regardless, thank you.