For the last couple of days I have played around with various programs, in order to set up a satisfying startup order & procedure. One negative side effect has been that my screensaver suddenly became unstable. I un-installed and re-installed, but in vain; it was as if it was trying to use more memory than available. But it is difficult to alter a screensaver's priority; when the file is running, you can't use any other program!
After several hours I had the idea to add C:\Windows\Endless-Slideshow.scr to Process Lasso's "game mode", and voila!, the problem was solved. I cannot see any problems with a screensaver added to game mode; my desktop pc is not being "used" when the screensaver is running.
Just a micro tip ;-)