Have you tried reverting back to VGA to see if it still happens? If it does, then your vid card is messing up. If it doesnt then it could just be the HDMI cable you bought. You might hav got a faulty one or the connector on it is bad. I am putting money on it being the HDMI connection. Make sure all the connections are tight as well. Cause if its just a tad lose it be moving around enough to be doing that.
Edit: Oh and make sure you have the latest drivers supported by MxO not the vid card itself. Countless times have I upgraded just to have MxO issues and hve to rollback.