OK, let's try this one more time. You want kde3 behavior, correct?
In Configure Your Computer:
Uncheck Let power devil manage screen powersaving
2) Advanced users settings
3) Computer Administration
Uncheck Enable display power management
-- or --
Check Enable display power management and configure it to suit your needs
I strongly advise doing it this way, because it is extremely easy to miss a setting in PowerDevil (If, for example, you wanted to set all schemes to Performance, you must do it for Screen, for CPU, edit the Actions Profile... ad nauseum -- a real pain)
I hope this clarifies my previous post -- it really should work. If it doesn't, I would definitely begin to suspect a conflict with some other installed package, a slightly borked installation, or worse, an intermittent hardware failure. Fortunately, the most probable situation is that you tried to use PowerDevil plus Enable display power management and you just missed missed changing a setting somewhere in the labyrinth of PowerDevil.