I've had this problem as well and I haven't found a fix other than to take 'em off my bike and do without 'em. IMO, the hinge design is sucky and replacement might not fix it. I had the pin on the left armrest work its way out on a ride on a really rough chip & seal road, and after the LBS replaced the hinge assembly it worked okay -- for a while. Later that year I found myself on that same stretch of road (the shoulder of TX-36 north of Sealy) and danged if the left armrest didn't start rattling itself apart again! It really ruins the ride when you're having to worry about part of your bike falling off, especially when you're tucked down on an aerobar you can no longer trust. My theory is that certain road surfaces, thus certain frequencies of 'rattle', will cause these things to self-destruct.
I haven't bothered to put them back on my bike, and if I ever do I'll experiment with JB Weld or something similar to keep that farking pin in place...
