Assuming the picture quality is good and not grainy on whatever you can see of the picture, it sounds like it could well be dry joints or bad capacitor the frame output (vertical output) IC or the associated circuits.
Dry joints to frame output ic's are very common as they tend to run quite warm and are almost always attached to a heatsink. Usually 7 or 9 pins.
Very common and very simple repair for an engineer so don't throw it through your window just yet. It will only get worse if left and you'll probably end up with total field collapse (horizontal white line).
I would usually replace the IC anyway (they are cheap and it ensures a reliable repair) and check electrolytics for low capacitance around it, but you might get away with simply resoldering the IC.
Otherwise bad vertical sync. could also be caused by poor voltage regulation (including hum bars due to poor mains smoothing), tuner or I.F. faults. But as the bashing seems to fix it, it's probably the frame IC dry jointed.
Cobweb, July 2006
