If they go away after the computer's done thinking, it just sounds to me like your processor can't handle painting windows and loading programs at the same time.
Look into your display adapter - perhaps there are optimizations you can set to shift some of the load to the GPU. Although, with a system that old, it probably doesn't HAVE a gpu.
KDE is very hungry. You should probably switch to a much lighter window manager, like IceWM. on a 166Mhz box, every cycle counts.