New Briggs 13.5 HP runs rough at mid engine speeds

I have a new Briggs XR 13.5HP 420cc engine (1.5 hours so far).  It starts fine and runs good at it's lowest idle and a little above, it also runs good at full throttle and just below.  The problem is at the RPMs in between it runs rough and rattles the machine it's on. I've had many Briggs engines on all kinds of equipment and never had this issue with any of those.
 
Has anyone else seen this problem before?  Any ideas on a fix?