Check your starter bolts to make sure they're nice and tight... I've seen them loosen a bit over the years, and the starter stops making a solid "catch" on the flywheel.
It could also be your starter itself getting old/tired/dirty. You can try taking it off and cleaning it so it engages more easily.
Or, it could be that you have a couple of ground/chipped teeth on your flywheel... it's not unusual either.
