I'm currently riding an old Rampar, with all original parts. Derailers are SunTour. I bought the bike in early August and started commuting heavily with it toward the end of September. Not long into riding this bike, one of the chain links became stiff. I thought nothing of it, given the fact that the chain was about 30 years old, and had not been used very frequently. So I replaced the chain. Or, more accurately, had it replaced.
For about 900 miles of commuting over the last two months, I had no problem with the new chain at all. I rode the bike in all conditions including rain, snow, and sand leftover from the roads being taken care of. Just yesterday, one of the links on my new chain became stiff.
My question is this: what is the cause of these stiff links, and how often should they really happen? I typically lube my chain once a month or so, depending on how often I ride.