After my desktop has been in operation - 20-30 minutes (running very well until after that);the fonts will become unreadable, as soon as I right click - that menu is unreadable as is the usual menu. Icons in the menu list(s) are jumbled.

Occasionally I will get diagonal lines upcoming down from the corner of the screen.
Have tried:

refresh
dcop kwin KWinInterface refresh &>/dev/null

Any suggestions or else Ideas?