The GTAC support is outstanding. Follow their advice first. Failing that, I'd move to MSTP. In the Cisco world, every time you create a VLAN, you create a spanning tree instance to go with it even though you may only have one physical topology. Any implementation of PVST is inefficient at best. On the last job I had, we were integrating a large Cisco network with a smaller multiple-vendor network. MSTP was our answer. Although MSTP will require more configuration (try using NetSight), the industry standard implementation of spanning tree will work with all platforms. Good luck.
Marcus