I had this exact problem with my shafty and eventually it got sooo bad I
donated it to a bike co-op. If you caught it the first time it happened and
stopped riding it right away, you might be OK. If this phenomenon
happened a few times you are in bad shape, 'cuz the gear teeth actually
get ground away. Your Dad is correct (but you knew that ).....The screws
are not nearly enough to take the strain of the shaft. Its bad design, period.
I would suggest taking it (BB)apart and loading it with grease. Then, blue LocTite
EVERYTHING before tightening it up. Goop up the cones that go into the bottom
bracket where they meet the shell of the BB bracket on the frame and also the
three screws on each side of those cones that tighten them to the bottom bracket.
If you caught it before steel got chipped off the cogs, you'll be OK.
Also, on mine, the crank arms were very soft and also required constant tightening
to the axles. Good luck !