screen-lock doesn't appear to work properly F18, F19 KDE

[DB]

Good evening all - at least, it is evening here in Austria!

My 2nd machine is a Toshiba Equium A60-692, with a (massive!) 40GB 
harddrive & a (minute!) 750MB RAM.

It used to work fine, then I tried to fedup from F17 to F18......

Most of the time it would work fine, as long as I kept the mouse moving. 
  If I let the mouse stand still for too long, the screen would go black 
& a couple of seconds later the cursor would vanish & I'd have to do a 
power down restart to get the system back.  Same story if I did a 
right-click & lock screen - no wa to get the screen back.

Tried a fresh install of F18, same result; fresh install of F19 same 
result (all with KDE)

Tried a fresh install of XFCE F19 Live,  everything works fine.

Does anyone have any idea what might be stopping screen-lock  from 
working?  Or how to go about finding out what is really happening???

As ever, many thanks for any ideas!!

Dave