XDCAM-EX playback slow and strobes and leads to crash

I'm editing XDCAM-EX footage on a new 2.4 GHz Intel Core i7 with 8GB memory tethered to a thunderbolt display. My hard drives are on 800 firewire. My playback consistantly strobes and as the day winds on it gets slower until I have to reboot at which point when powering down the system always crashes and I have to do a hard restart. Playback is fine for about 15 minutes after start up.

I'm really enjoying the CS6 interface but considering returning to FCP7 or AVID with this project so I can SEE and HEAR what I'm cutting!

Anyone having similar issues?

Patrick

ps Mercury playback is on & 6GB are set aside for Premiere.