[Darryl Ramm]

Could be a classic sign of a plugin memory leak (which can happen with no session open).

How much of the disk cache is actually used? How large are your sessions? How many/what VIs -- the classic memory pig plugins, but if it's a leak any of them can cause that.

Does this happen only with one session?

Check all your plugins are up to date, as always start with trashing prefs did that fix it? If not try removing all .aaxplugin files from your plugins folder and trash prefs again and restart and just play around with only the default plugins (that get put back magically by Pro Tools) installed. Add others back in batches to see if you can find which one is causing the problem.

You also need to be a bit careful with looking at memory stats, things don't always work how they might seem. e.g. The amount of apparent free system memory can to run low in a healthy system, its the virtual memory system just not being bothered to spend effort freeing pages (which if read from disk *are* the systems disk cache for programs and data) or flushing stuff back to disk if it does not need to. Often low-level tools give more insight than the simple meters.