Not sure if anybody else has mentioned this or not, but in addition to the air horns, do you have anything such as filters or an airbox that lessens the air turbulence around the horns?
I have 40s on my car, and if I drive it without the special air filter and housing I made for it, and just the bare horns, it will stutter and stumble upon acceleration, and not accelerate nearly as smoothly as with the filter and lids. Plus, it will pop and backfire on deceleration.
I used an early 240Z filter, sandwiched between two aluminum plates (just like the smaller ones but combined, with a larger filter. I needed to crush part of the filter and housing to make it clear the brake booster. Note that this was in 1997, long time ago!
I also have a metal bracket that runs from a bolt on the starter to the center of the housing back plate to keep it rigid.
Hope some of this helps,
vince