Hi,
I'd say it's a power supply problem for the horizontal deflection part.
It may be a GND connection problem, too...in the horizontal deflection area.
Use an isolating stick to gently move some wires...while you look on the screen (with a mirror) to see if something changes.
If it changes, then it's most probably a bad connection...try to locate it...
If nothing changes, then check the power supply for bad (old, bloated) electrolytics capacitors.
But maybe it's someething different...
Klaus