I've said this before but it bears repeating:
Periodically format the card in the recorder, no matter what card you're using. In my experience SD cards are fickle things and can easily develop filesystem errors and/or bad sectors. This kind of memory can and often does develop dead zones. This could happen to any card, any size, any time. A format every so often can help these issues by rebuilding said filesystem around those places.
Definitely follow the recommended media lists, but don't be surprised if you still encounter issues like this on recommended cards. It's the nature of the beast.
I also have a theory on why we are possibly seeing this with the 70D more than we have in other recorders: Due to its price, it may likely be the first 4-channel recorder many of us have owned that records to SD cards. Recording in 4 channels is going to be more taxing on the card, making it more likely that you'll encounter these sort of issues.