One thing to try is to see if there's any "play" in the bottom bracket. You can check this by grabbing a pedal and trying to shake the bike. Also, do you hear any bearing noises when you spin the pedals?

Also, you could check see if the headset needs attention. Can you easily steer the front tire without hearing any scraping?

These might be fairly troublesome repairs for an old French bike.

I was looking at an old 70s Peugeot yesterday. It had cottered cranks (yours don't...) and steel rims (not sure about yours...) which told me it was fairly low end. It weighed about 35 pounds too. However, it also struck me as being bomb-proof...