Been recently having this problem, where artifacts appear on the bottom of certain renders (see examples below). They only appear on certain renders (so far, only ones with a transparent image overlayed over the video). They do not appear in premiere pro or the raw footage, only on the exported files.
Since they effect the overlay as well as the central main video (see second example) I'm fairly confident that it's not an issue with the source footage. Source footage is AVI captured from fraps with transparent PNG overlay, desired output is h.264 (PRPRo's Youtube Widescreen HD preset).
My initial thoughts is that it may be a processor issue. My processor is a little on the weird side, and I have an upgrade coming in the next few days, which I hope may fix it. Current processor is an AMD Phenom 9950 Quad-core @ 2.60GHz, and my GPU is an NVIDIA GeForce GTX 460. Thanks for reading, and I hope you can help! If anything is unclear, let me know, and I'll try to clarify.