a screensaver is just an .EXE file with a different extension (try it!)
many "free" ones contain keyloggers or other malware .... genuinely free ones are often created by inexperienced ppl
and they all to some extent conflict with the system's normal operation (a sort of mini suspend).
in every case they hog resources by constantly monitoring the system for idle periods
modern screens don't suffer from phosphor burns .
and - if you think about it .... why do you care what's on screen ... when you aren't there ?
If you are away for a great length of time ... the best screen saver is to SWITCH OFF!
the only vague excuse is security .... and the windows screensaver is ideally suited to that as it can be set to need "propper" logon details.
without knowing about your system, I suggest you check that you have the latest graphics drivers for your graphics card, If you have and its still happening then check you have direct X, if still no joy then your graphics card is probably a bit old and naff :D