JVC HM700 invalid media with Kingston cards

We're having an issue with our JVC HM700 cameras, so far only when using Kingston class 6 SDHC cards. While recording, an "invalid media" error will show up, causing camera to switch slots to other card or stop recording entirely. We have experienced the problem with two different cameras.

Wondering if anyone else has had this problem, specifically with Kingston brand. It does seem that the cards do not fit as well in the slots, so we're wondering if they are not making proper contact. Our purchaser is having a hard time locating SanDisk, Toshiba or Panasonic brands in 8 and 16 gig sizes, at less than twice the price. We are currently archiving the cards themselves, and bound by government purchasing bureaucracy, so cost tends to be a deciding factor...(but if this is a known issue, we could exclude incompatible brands from future buying!)

Thanks for any help!
Alan