Does it feel like the transmission is kicking the shifter out of gear? I've had this happen with my LX quite a few times, but it's also happened in all the manual transmission vehicles I've own(not that three is a lot, but it has happened in all of them). My first was a '91 S10, my second was a '93 Civic CX hatch, and now my '12 LX. Not really sure what would cause it though. Maybe the synchros are in such a position at what ever RPM you're at that it'll just kick it out of gear. And this was whether or not the A/C was on.