Entire frame or an artifact spot in one of the frames? Can you view it somewhere frame by frame, stop frame on the red flash frame?
Generally I've found unexpected artifacts are **usually** due to something on my system being goofy. The closest example I can recall ATM is before I added another RAM chip and larger HD, I used to get entire 2 or 3 seconds of corrupted video data, like static or misplaced parts of the frames. Re-encoding it with all other applications closed seemed to help, which was my clue.
You can also use the encoder as a standalone without having to open the extremely memory intensive application (Premiere.) The encoder should install as a stand alone when you installed Premiere.