Heavy hesitation on highway

Michael Handzel

Mike`s XS400
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I have a 1981 Yamaha XS400H with a little over 11k on it . once on the way to work (a 20 mile trip) the bike started cutting out and hesitating like it was going to die. Freaking out , not wanting to be stuck on the side of the road , I held the throttle WIDE OPEN and when I was almost to my exit, it cleared out and ran like it never happened! there's a slight downgrade where it started clearing out (less load on motor?) that was two months ago.
Yesterday on way to work it acted like it was ready to go on reserve , (you know the hesitation you feel for that) So I put it on reserve. The Odometer reset does not work. So on way home yesterday about halfway home, it started the Hesitation Real bad again. tried Choke, turning from on position on petcock to reserve, to no avail. Even tapped on , more like hammered on gas cap , trying to see if it was vapor locked from clogged vent.Finally the last hill down to my exit ,it cleared up like whatever was in system passed! Rode fine on City road rest of trip home. Stopped and got gas ,took 3.5 gal.
Rode to work today,bike performed Flawlessly.???
Certainly sounds like a fueling issue. You may want to check the carbs for dirt/debris as well as the tank petcock. It should be a vacuum petcock, so maybe the diaphragm inside it is starting to fail?