Uneven video speed using Kodak Zi8

[spoolin01]

I'm a noob with video so I don't know if this is a camera problem or what. I've been using the Kodak Zi8 in the Ikelite housing and I regularly get what looks like a 'revving and slowing' of the video speed. A smoothly moving subject appears to go faster/slower/faster/slower in a regular rhythmic cycle, about 1 second fast, 1 slow. I switched from a SanDisk Class 4 SDHC card to an ADATA Class 6 and I thought for awhile that improved the situation, but it's still pretty noticeable. It seems to happen with or without EIS - but anyway there's no blur, and I've got a good two-hand hold on the camera tray in calm water. It happens even when I'm panning only slowly to follow motion. I haven't spent a lot of time inspecting all the clips - I'm diving instead! - but I'd say it's present in every clip I've looked for it in.

Is this par for this type of camera? Any ideas what is going on?

Forgot to mention, I'm shooting 720p/60fps. Haven't tried any other formats...