From what you posted, it sounds as if you're having issues with 120hz Eyefinity with the new 14.1 BETA drivers, correct?
Personally, I've had nothing but issues with the 14.1 drivers and have been running on the 13.11 beta 9.2 till this day (for various reasons).
As far as I'm aware, users running Crossfire should stay away from the 14.1 BETA drivers for now (regardless of using Mantle or not), until the Catalayst team releases a WHQL or something of the like.
Have you tried revertting back to a previous driver version (preferable a WHQL version or 13.11 beta 9.2) and see if the problem persists?
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