Had same problem befor I replaced the belt with a new one. The sides wear and reduce the diameter of the sheaves at any given position.z Belt looks good but is not wide enough. Mine went from 900 rpm max to 1200rpm
Another item to check is the bottom sheaves (driven) on the fan itself. Take it apart and clean it properly. Sometimes they get over-greased and get sticky with dust,dirt, etc and do not move the way they should.