Again try to pump the primer bulb when it starts to die. I had one do something like this years ago and it was a timming issue.
Fallow along;
Motor would start and run great up to 3/4 throttle, at WOT (full throttle) it would run for about a mile then the motor would slow on its own, start to pop and missfire, then quit.
Motor ran great as long as you did not run to WOT.
While testing we watched timming with a light at WOT we noticed the timming would retard by itself until issue would again happen. As long as we kept below WOT timming would not move.
After motor quit if we tried to restart ASAP it would pop etc. we let set for 5 min, it started and ran normal.
The issue was a faulty bonding material betwen magent and flywheel surface alowing magent to slide at WOT on flywheel. Replaced flywheel issue resolved, again this is a long shot.