Hi Digger, I don't use 11 but I presume it's pretty similar to 10. The only thing I've found works is to either lower the preview quality or pre-render the clip. Now that you have more RAM you could try experimenting with the Dynamic RAM Preview setting, on the Video tab of the preferences, to see if that helps but I've never found that it does.
The only other advice I can think of is to make sure you have the most up to date version of the 64bit build of Vegas.
Sorry if that's rubbish advice I'm just not a Vegas settings expert.