Framerate drops to 40, profile says 60% in 'Overhead'

Hi, our game runs at 60fps, however at certain points this drops to 40. There's no in-between rate, it's either 60 or 40. When at 40 the profiler is showing 60% cpu time is spend in "Overhead". Image included shows the transition from 60 to 40:

The Usage shows "Others" taking a huge amount of CPU every other frame, causing the overall fps to be 40.

• Unity 4.6.8p3

• iPad Air

• iOs: 9.0.2

Questions are:

Has anyone else experienced this?

How can we drill down into Others or Overhead to narrow the cause of the slowdown?

If there's some throttling or energy saving mode occurring, is there some mechanism for disabling it?

Many thanks.