Sliced and confused

Ok....I have a Callaway Hyper X 10 degree driver with a grafalloy pro launch blue shaft 65g stiff shaft.  I was fitted by a local shop for the shaft.  Problem is, I slice the crap out of it.  I got out an old driver we had in the garage and took it to the course the other day.  Hit it straight and better distance.  It is a Great Big Bertha II 11 degree with a UST HTD extra stiff shaft.  I wasn't fitted for this shaft.  It just had that shaft when we bought it used.  I took the two to the shop and hit on the launch monitor and was consistently straighter and about 30 yards longer.  The confusing part is my swing speed (95-98) really isn't high enough for the extra stiff shaft, but I hit it straight.

Any thoughts?

You hit it straighter because a stiffer shaft kicks less. If you have a stronger swing, you'll bend a less stiff shaft more. It'll actually kick at the ball earlier and more vigorously than a stiffer shaft will. Aka, stiffer shaft, less amplified misses. Odds are the x flex shaft is still pretty stiff even though it's been sitting in your garage and your swing speed isn't too low to benefit from it.

Well that was my thoughts too but I read several things in researching it that said stiffer shaft results in slice.  One reason I was so confused.  I really want to put that shaft in my hyper x.  Only problem is what if that doesn't work same as the big bertha II and I have to go pay to have switched back?