I don't know if it apply here, but (reliable) class 4 cards are generally *much* better at random access/writing/reading, but class 6/10 cards are superior in bulk write/read.

HD recording seems to be the second situation, so it's best to have 2 cards around - class 4 for swap on microSD, and class 10 for HD recording. If this is really the source of stuttering...