I think that I found the same difficulty with the live disk in that the changed resolution didn't want to remain. However, once I had installed to the hard disk, I changed the resolution in PC Control Center - not KDE Control Center.
The displayed settings in PCC > Hardware > Configure video card are:
Graphic Card Intel 810 and later
Monitor Flat Panel 1366 X 768
Resolution 1366 X 768 24 bpp
These settings now operate each time on bootup.
So Canuck, I think you will be able to resolve the problem there. Good luck,