More than likely, a stumble (i.e. the car loosing power,) is due to carburetors (the second most likely cause is timing/ignition.)

Stumble or dying on long hard corners

Float height is probably not set properly (or your floats aren't working properly.) The bowls fill with fuel and when you aren't pushing lateral G's the engine gets flooded.

It has also been suggested that fuel can be getting in the air correction for the idle jets. The proposed solution is to drill holes from the banjo bolt location towards each idle jet.