My monitor is a dell U2713hm. I'm using the displayport but I tried DVI as well and the result is the same.
I'm thinking it may have to do with the refresh rate and how it is detected. My previous cars were nvidia and used to detect my refresh rate as 59.95hz vs 60hz for the ati.
I read somewhere that if the drivers weren't able to maintain that vsync, they dropped to half the refresh rate for some odd reason.
If it's reading the refresh rate wrong, that could be the effect.
Intel Core i7-4930k
32GB Ballistix Tactical LP 1600MHZ
MSI Gaming R9 290 CrossFireX