1998 Buick Century Windshield Wipers

I have a 1998 Buick Centruy with (roughly) 181K miles. A few weeks ago, my windshield wipers started having a problem. When I initially turn on the wipers, they are working correctly, however, with the subsequent cycle, the passenger side wiper begins "lagging", i.e. it doesn't rotate/swing across the windshield as far as it should. After a few more cycles, it reaches its maximum "lagging" stage and at the end of each cycle, it returns the the park position. It continues to move "back and forth" at the correct timing intervals, but it continually fails to rotate (on the outward bound sweep) fully and it always return to the park position at the end of the return sweep. What should I check?