I recently bought a vintage peugeot iserna its and 86. The bike is in great condition and the gears worked well everything did. I had to switch the tire out both front and rear as they were dry rotted. after I did this the rear gears automatically keep shifting down untill the chain pops off. I am guessing i have a problem with my derailer. if Anyone has some tips on how to fix the problem or knows what may be wrong please help me out I would love to get this bike out on the road.