For what it's worth, Rod Jenkins, Rick Welch and a number of others I've talked to suggest holding the bow lightly, with a 45 degree grip - if you wrap your hand around the grip and then try to choke the bow, you'll torque to the left or right depending on which side you shoot from.
So my best guess is you're getting riser slap because your torquing your riser.
Try going back to your old grip and then if the string is slapping your arm, raise your brace height. Odds are your brace is too low, as Michigan Bill said.