I've been shooting video with no problems using a SanDisk Ultra Plus 16GB 80MB/s SD card and recently picked up a second one. The exact same card.
On the first one, I can shoot as long as I need. The new one I bought stops after 5-10 seconds. I tried formatting it in the camera and on my PC without any luck. I also bought a few Lexar Professional 32GB 95 MB/s cards and they stop after a couple seconds as well, even after being formatted.
Any other suggestions for things I can try? I'd really prefer not to have to return these and buy CF cards but I'm losing hope and have an event next weekend....
Try using the Sandisk Extreme SD cards. You are using one of the cheapest cards they make. Be sure to do a low level format in the camera with any card that you use, too.
I would suspect a counterfeit SD card if it is not performing up to spec. Lots of counterfeit cards out there.
I suspected an off brand card. Have not ruled out a counterfeit card, either.