Bobhh , I had a similar problem with a bipe with an OSFS120SP supercharged engine.This engine has a gear pump and a pressure regulator before the carb. It also used to either cut or sag and blow a trail of smoke then recover after violent manuevers. Tested engine on bench , and used the same tank and fuel line from the plane. No problems! I shook the tank to simulate vibration and sudden manuevers , big problem! Turns out that when i shake the tank i,m getting massive fuel foaming which the pump does not like. Engine would sag blow smoke as if it were rich which in reality is massive leanout under load causing instant vaporisation of my oil fuel mix(mix my own fuel) The fix was simple i started adding a couple of drops of Armour oil vinyl protectant to my 4 litre fuel jug on completion of fuel mixing.This stuff contains silicone which helps lower the surface tension of the fuel virtually eliminating the foaming. Guess 'cause i mix my own fuel that i'm missing some of the additives of commercially produced fuel. I also added extra vibration absorbtion mounting/padding to my fuel tank (mounted on C.G ) to dampen a bit more of the vibration from the airframe. Test flew the next weekend , result , best flight i've ever had with my CG Ultimate! The engine ran basically as it ran on the bench , awesome top end and excellent idle , not one hiccup since. Just my two cents , but id try looking for vibration isolation of the tank or an anti foaming additive to your fuel if you mix it yourself.These fuel pumps have temendous flow and pressure but have very little tolerance to aerated fuel and you can't see that happening in flight !Leon.