Odd frame time spikes since upgrading (R9 290X)

I upgraded my main desktop a few weeks back; I wanted to get some AMD cards to play around with Mantle. After the hardware swap I'm getting periodic spikes in the frametime that are pretty disruptive to gameplay. These stutters didn't happen while the machine was running nVidia hardware. Is there a known issue about this sort of thing? With the hardware in question there's zero reason why POE shouldn't be running pegged at 60 fps much like it was on the older hardware.

System info:
Intel 4930K CPU on an ASUS X79 Deluxe and 32 GB of RAM
Samsung 840 Pro SSD boot drive, game running off a larger Samsung 840 SSD.
Windows 8.1 Pro 64-bit
Game is running in fullscreen windowed mode on an older 30" HP 2560x1600 monitor. All in-game settings identical between both cards. No AA/AF settings forced in the driver.

AMD-specific (stuttering) setup:
2x ASUS Radeon R9 290X graphics cards running in Crossfire
Catalyst 13.11 drivers

nVidia-specific (older but not stuttering) setup:
2x eVGA GeForce GTX 680 cards
GeForce 331.82 drivers


Here's a screenshot with the information graphs shown:


This happens with some regularity, at least a few times a minute. It's particularly bad around shrines.

Have you found any solutions or tweakes ? i am using r90x as well.