If you're using Hi-8 tape, that's at the core of your problems. The inertia of it's mass (actually more the mass of the heads) can cause your tape and heads will go out of sync during recording from vibrations. Upon playback, the signal will roll in and out of sync. MiniDV cams suffer less from this due to their smaller tape and heads, but they too can suffer from this.
Granted, you can reduce it somewhat, and fortunately you've found a sweet spot to get better tracking, but once you can record from a solid state device, your footage will look a lot smoother.
Solid state cams are going pretty cheap nowadays, you may want to look around and get some user feedback on various models. The cheapest of them will render bad MPEG (which won't look much better than what you've been getting), but mid-to-higher end will use better algorithms and get better motion tracking.
Bad high-speed motion tracking has traditionally been the bane of MPEG video, but things have improved. Some cameras will even record to other formats like standard DV direct to memory.