a cpu overload error occurred - help:=)

Hi!

I have an imac os 10.8.4 21.5 inch late 2013

avid mbox 4*4 1.1.4

And pro tools express 10.3.4



It says on avids site that this two are compatible.

My problem is this error occurring saying.

A cpu error overload occurs. If this happens often, try increasing the H/W Buffer Size in the playback engine dialog, or removing some plugins (-6101.)



I have tried to increase the buffer size and also tic the box to ignore this error windows. The message even occur when i have no track recorded.

Sometimes it is okay for 15 minutes and suddenly its back up again. I have had someone over to look at it, but he found no solution.

Any one knows this problem.

Thanks from Cathrine