10-10-13, 08:35 PM
Tired cush drive rubbers would add to the slop you feel when rolling on and off the throttle but wouldn't give the O-rings a harder time.
Misalignment of the sprockets probably isn't a likely cause either… if they were out by a long way the uneven strain would cause the chain to wear faster, but the O-rings ought to survive until the chain is noticeably worn. Still worth checking the run though, the old sprockets will show heavy wear on one side if there is a problem.
The missing O's aren't all from the soft link by any chance? If the side plate was pressed on too hard when the chain was installed it could be that those O-rings were under too much pressure (in which case a new soft link will solve the problem cheaply).