If you use continuous Hi often then I would look at getting a faster card. The Lexar Platinum II 100x is a good card but there are faster ones that won't bottleneck your shots as much. I've had that happen to me before. On continuous hi you really eat away at your buffer. All of a sudden, once your buffer is full the camera won't allow you to turn off, actuate, focus, etc.
As you take pictures you can see in the bottom right corner of the viewfinder an "r" along with a number. That number represents the estimated amount of pictures you have in your buffer. This number fluctuates as you take pictures (goes higher as memory is able to write to card and goes lower as you snap off pictures faster than it can write).