Sluggish after long rest

[mreg376]

My iMac basically works perfectly, I love it, I have converted other people, and I have had no complaints in my 20 months since conversion from Windows to OS X.

Except one.

The only power saving feature I use is sleeping the display -- otherwise the machine is on 24/7, and the only time it ever needs to be rebooted is after a software upgrade requires it. However, if I'm gone for several days, when I return and I move the mouse to wake the display the machine is VERY sluggish, as if it's working under water. Everything. At first I thought it was Fusion, but it happens without it. Istat shows nothing unusual. The only solution is a restart, and then everything's fine. Again, this happens only after the machine is left alone for several days.

Also, this occurred with Leopard and occurs now with Snow Leopard.

Any help would be appreciated.