Weird FPS issue

I play at 720p because i happen to think the game looks better at this resolution than 1080p but today i experienced a very weird problem.
When playing at 720p as usual the FPS suddenly dropped from a solid 60 fps and locked at 29 fps upon entering Uldah.
After messing around with the refresh settings which are set to 60 it made no difference.
Dungeons are solid 60fps areas still but populated areas lock at 29fps,previously there were only between 5-10 fps drops.
Oddly though if i up the resolution to 1080p,i hit a solid 60fps pretty much everywhere.
Any ideas folks?
I5 4690k@3.5ghz
GTX 970
8Gb RAM@1600Mhz
latrst Nvidia driver and latest build of Windows 10 with Creator update