....
should I change the flat belt to a ribbed belt - it wouldn't be hard to
put the pulleys on my lathe and turn grooves in them to match a grooved
pulley, but if I did that there would be no going back to the old flat
belt - and I would loose a safety factor

Second Q - would a new flat belt make any difference?


I wouldn't change the design. Flat belts with all good components are
reliable. You might want the double check that your flat belt pulleys are
crowned, or that the crown hasn't wore off. This is what keeps the flat belt
from tracking off.

Karl