RenderOverlays spikes and stays up?

Hi, I am building an app for iOS, based entirely on UGUI, but am having some issues.

I have added this to Start()
I have set the QualitySettings.vSyncCount to 0.
Environment.SetEnvironmentVariable("MONO_REFLECTION_SERIALIZER", "yes");

I have set my target to 60fps

In Update() I have

Code (CSharp):

1. if(Application.targetFrameRate < 60){
2.      Application.targetFrameRate = 60;
3. }

This works most of the time, but sometimes, not sure why, jumps and spikes to 33fps and stays there.
I have AdMob banner ad in the bottom which seems to be the one triggering this, course if I just keep the app open until the banner changes it falls int the right fps again. Also, if I pauses the app and starts it again I can get it back to the right framerate.

My question is:
Is it possible to force the farmerate back given this issue. Is there a AdMob fix (if it is AdMob triggering this)?

Really hoping for help on this issue and thanks in advance