It isn't the Magicshine.
I tried moving the Blaze farther away from the computer, so I had only it on. While the bike was in the rack, I was going 38 miles/hour.
I've had 3-4 different light combinations, all including the Magicshine, but only the Blaze is interfering with the computer.
I haven't tried the foil. I'm not sure how often I'd have to mess with it to keep it on.
For now I just turned the computer off and I'm using the Endomondo Sports tracker app to track my miles.