What graphics card is in that system? I have one with an Nvidia card that does similar – I had to put a script in ~/.kde/Autorun to run something like:
nvidia-settings —config=~/.nvidia-settings-rc —load-config-only
After having saved the settings I want using the nvidia-settings util
(Note that the —‘s should instead be two dashes)

(I’m not using that PC right now so it might not exact)

This doesn’t answer Why, except that it’s a closed-source driver so it’s up to Nvidia to fix bugs (on my system at least)

As far as the main panel, I don’t have much advice, I’ve generally seen it say as a percentage of the screen, so when the screen changes it squashes or stretches.

(And if this answer was even slightly helpful, please mark it as a Great Answer – I am trying for a couple of the awards ;-) )