Riccardo,
Have you isolated the problems you are having to VW application only? I noticed you said no other programs were running, and some of the issues you stated sounded like they might be a hardware issue (in conjunction with VW or some other conflicting software).
You also stated the machines were just recently updated to OSX 10.2.2, I had crashes about every hour and slow screen redraws etc. prior to updating to X10.2.3 and VW 10.0.1.
Also prior to updating the OS on those machines, were they checked for a firmware update (the dual probably doesn't need it, but the 400 might) as well as the superdrive update (if the dual is so equipped)?
The 10.2.3 update can be accessed via System Preferences/software update. I saw an immediate difference in VW. I hope this may have helped some.
I ran accross my old drafting machine in my attic this weekend, and after thinking long and hard about the way I used to do it........Crash? What Crash? Those are my automatic "Please check your email a couple of websites and maybe get a soda" notifications built in to my computer!
"Dont' hate the player, just hate the game" - Will Smith
Matt