1991 R100RS: The rear suspension feels like it is bottoming out sometimes, not all the time. At the beginning of a ride (an hour ride) the rear shock feels fine but by the end of the ride, it occasionally feels like it bottoms out (clunk) when I hit something like a speed bump. I'll try to do it on the next bump (same size bump, same speed), but without success (it feels normal). So, it only does it sometimes. Is this a symptom of an aging rear shock? The bike has 35K, I cant find anything obviously loose. Any help? Thanks!