Well...interesting to me that it appeared to happen at mid-throttle range (if I read the post properly?). I wonder if the lower cylinders are actually firing at that lower rpm, my no.3 cylinder appears to drop out at idle and chimes in with higher rpm. If the cylinder is not firing could fuel build up? Wouldn't it drain when the valves open? Perhaps not, certainly not on the compression stroke.
I guess I'm just wondering if it's an overfuelling problem on the lower cylinders at lower rpm's? At lower rpm's it can be difficult to tell if a cylinder has dropped out, other than a drop in temp on that cylinder.