Having had extreme annoyance

Having had extreme annoyance - sort of like an itch that you itch that makes it itch more - My screen flicker has seemingly gone out of control.  I rebuilt the computer, am now on 10.5.8.  This helped for a few minutes.  I looked around - and came to the unfounded conclusion that it was most likely not a hardware problem.  I have had replaced two batteries, one charger, logic board, and the cases (AppleCare.)  Today, I stumbled into "safe boot" which does all the stuff mentioned in the following Apple document - What is Safe Boot, Safe Mode?  - ¿qué son el inicio seguro y el modo seguro?  -.  And, of course, it's best not forget the "Shortcuts for starting up" in the Apple help menu. Might help.