Stephane,
I think you have an air pressure problem in flight, the pump regulator and carb intake must have the same atmospheric pressure. If your carb inlet or regulator inlet in the slipstream or receiving ram air and the other is not then an imbalance will occur and will drive you nuts. Try using an extension tube or placing a cowl in front of slipstream airflow and your problem should be solved.

Ciao,
Jerry