I'm using a class 10 8GB Kingmax card, and it's been great. Zippy and stable.
I'm going to try mine again with the newer image and if that doesn't help, go back and exchange it for a Kingston. Most of my cards here are Sandsisk and Kingston -- the Class 4 is a Kingston. My newer one is Patriot, which is a brand I've not had any problems with when it comes to notebook memory and they were the only brand in stock that had a 16GB option.