what is tha supercharger fitted to? If it is not a standard fit up check to see if the belt is slipping. had one doing the same thing on a g200 gemini, found that when i lifted the foot to deccel the belt would slip as the sc was still making boost but the engine didnt need it so it would try and stall the sc. the problem was fixed by fitting an over boost valve. if it is a standard fitment i can only suggest you check the over boost valve which will be on the intake plenum some where