It's strange because I have a similar issue with only my balance druid and shadow priest. All my dps share the same profile (not the exact same, every dps has their own profile in the profiles list, but they are all identical copies), but only those two have those issues. I'm pretty sure at this point it isn't some random issue as shadow priests and balance druids are mentioned many times in this thread. My fps issues start when the health bar gets less than full, that is when in combat and taking damage.
Here's a link to the thread I made about it, in case someone wants to read more details.