I changed my mind assuming it is a lean stumble. A low float setting
and/or possibly an obstruction in the fuel delivery system would give a
low bowl level that could make the mixture go lean enough to stumble
from the main jet. In that case it would also be running lean at full
power - not a good situation.

This still doesn't fit every symptom. The cheapest way out is to get
an EGT - and soon..