Why is my Macbook Pro 2011 video garbled after sleep?
It was bad memory.  I had upgraded to 8GB, but at some point suspected the memory for some other problem, so put the original Apple 4GB back in.  Both problems went away (tested for 3 months) so I ditched the 8GB and got 16GB of higher quality memory that more closely matched the timing specifications of the original Apple memory.

So far, after two months, I haven't seen this or the other issue yet.  So in this case it was bad (or wrong) memory.