I have a couple of bikes with French thread, that have the same issue as you describe. The right side of the BB needs to be tightened to a very high torque, for it not to work itself loose. I didn't torque the BB enough once, it loosened up while I was riding. I then kept re-tightening the BB by hand every 300 meters or so, till I got to the shop with the proper tools, where I tightened the BB very hard - and never had a problem since. With English thread, you'll not have the loosening problem while you are riding, but it can be really difficult to remove the BB after a long time of riding, if it ever needs removal. The English thread keeps tightening the right side of the BB as you ride. There are many cries for help in this forum, where people are trying to remove the right side of the BB on older bikes without special tools.