On x13mod on a 290 rig i've found that my memory speed is bouncing between what I have it set at, 1250mhz, and 150mhz.  It continually drops to 150mhz and then regains to 1250mhz.  It happens roughly every 15seconds.  Still hashes, still submits, but this appears to be dropping hashing speed over time.

My 280x rig doesn't do this with same software.

Anybody else seeing this and any ideas?