It sounds to me like you're overpowering the shaft. Although, conventional wisdom says that because you're slicing, the shaft is too stiff for you

Move to a stiffer shaft.

A couple of quick rules:

1) If you're killing the demo club on the range, BUY THE CLUB IN YOUR HANDS.

2) Always consider an aftermarket shaft. You really can't trust the flex designation on the OEM shaft.