I've got a sick iMac - raster is full screen, but too wide with severe
pincushion. Monitors control panel has no effect for width, keystone or
pincushion, but the other adjustments work. Actually, if width is set at
the very leftmost (00), there is a tiny affect, and then keystone and
pincushion also have a tiny effect. But nowhere near enough to get the
raster straight and narrow enough. Same effect with Apple Display Utility.

PMU reset, PRAM reset, etc. I'm told this happened suddenly. This model
has no VGA output.