I fashioned an adjustable cover for the venturi by folding over a sheet of aluminum foil multiple times to form a strip, then wrapped that around the venturi intake. I can adjust it by sliding it back and forth to cover more or less of the opening. In this position, the flame is more consistently blue.
I won't leave this on overnight because we don't need hot water. I'll turn it on again tomorrow when the wind picks up to see if it addresses the problems.