I have the same lag issue. Strangely even with a class 6 card installed (that I've used for 720p recording on other devices with no issues). I have since picked up a class 10 card (which according to sd card tools tells me my phone can actually use class 10 speeds), and now the lag is gone. I haven't done longer recordings but at least 30 seconds in and there's no lag...unlike before the video lags behind the audio, then catches up every 5-10 seconds.
Technically if the phone writes directly to the SD card and doesn't store it locally then transfers the video to the SD card, class 2 is not sufficient for 720p video recording. I also found it strange that at a Tmobile kiosk I was at asking about an unrelated question, the representative pulled out the SD card from their demo unit and it was an 8gb class 4sd of the same brand...not sure what's up with that!