Close to 800GH/s?
Care to post a screenshot of CGMiner average speed?
Just for comparison - my 2 November Jupiters are eating 1770W from the wall with a peak of almost 2000W.
No immediate way to measure at the wall. Might invest in something to do that soon. If you are peaking at 2000W, that could explain why it would run for several minutes before clicking off.
800GH/s includes the Mercury using pool's measurement (varies between 750 and 850). CGMiner has me closer to 700 right now (585.3 + 123.9).