Reviewing my notes on this problem revealed that a frequent non-responding application was showing as SysFader. A Google search comes up with suggestions that this could be part of the Nvidia package or it could be a Windows process where it is known to cause application hangs. A suggested solution was to go to Display Properties - Appearance - Effects, and to remove the ticks from "Use the following transition effects for menus and toolbars", "Use the following methods to smooth edges of screen fonts", "Use large icons" and "Use shadows under menus". I'm not at all sure why any other than the first is related to SysFader, but I unticked them all anyway. So far it looks like I might have cracked the problem. I'll keep my fingers crossed and wait a few days before ticking this as resolved.