SPSX's are pretty "thin skinned". Are your barrels new and/or rough? If so, they won't work at any speed. I've been using them since they were first available and unless I ran them through a rough barrel or excessively fast in a tight twist, I've had very good luck with them. In a good barrel they'll do 3600 or so if the twist isn't faster than 1:14. One in twelve takes 'em to their "edge" in twist speed and anything else faster usually just causes them to 'disappear' such as is being described.
This was my thoughts, also.
