So, this is where we reach the limit of what I might be able to do remotely. The picture looks to be the effect of Horizontal size setting needing adjustment. If there are no other pot adjustment knobs then you may have to look for jumpers to make adjustments.
With that said have you plugged in a more current monitor to see if the effect goes away? Just to make sure it is not some setting in the Pi that is causing it.
below is a clipping from a crt trouble shooting site.
"Horizontal size should be set so that there is about 10-15 percent overscan left and right. This will allow ample margin for power line voltage fluctuations, component aging, and the reduction in raster size that may occur with some VCR special effects (fast play) modes.
Many sets no longer have any horizontal size adjustments and depend on accurate regulation of the voltage to the horizontal output stage to control horizontal size. There may be a B+ adjustment to perform first.
On those that do, the adjustment may either be done by setting the B+ voltage, by a pot, or a width coil in series with the horizontal deflection coils.
Modern sets do not generally have any linearity control but you may find this on older models. You will need to go back and forth between size and linearity as these adjustments are usually not independent.
Some of the newest sets control all these parameters via settings in non-volatile memory and use service menus accessed via the remote control for nearly all setup adjustments."
