I'm running it on my Mountain Lion with a dual-core 2.6 ghz and 4 gig of RAM and I've never had that problem, Stu. So it's really bizarre that you're seeing it on two different computers!
I assume you've run disc diagnostics to make sure it's not a disc problem. And you've run tests on the memory. And I also assume you've got at least 100 gigs of free space on your hard drives.
If so, the question I'd ask would be what do these two computers have in common? Do they have any of the same software?
Figuring out what your two computers have that mine does not will likely be the key to diagnosing your problem.
The first computer was running leopard. The current one is running Mountain Lion. Computer is two weeks old tomorrow. 732.00 GB of free hard drive. It is running really fast. I have not altered any of the preference settings in Premeire Elements 11 yet.
The only programs in common on both computers are:
Photoshop Elements 11
Premeire Elements 11
Spyder Pro 4
Are you running any of those programs as well?
Well, you can eliminate Photoshop Elements and Premiere Elements from that list.
For my money, I'd bet it's Lightroom causing the problems. But, short of removing it from one of your computers, I don't know any way to verify it.
Not knowing Mac's, this question might not be so easy to answer, but on a PC, one can look in Windows Task Manager, to see exactly what programs, services and processes are running, at any given moment. I would assume that the Mac has a similar utility. If it does, I would do a test, and quit PrE, after a Project Save, then look at that utility, to see exactly what programs, services or processes ARE still running, and especially which Adobe modules are running. That would narrow things down quickly. If the Mac does not have such a utility, there should be a 3rd party utility available to track down running programs, services and processes.