Since you said the problem started after the alternator was replaced and new belt was installed, are you absolutely sure the alternator pulley (on the new alternator) is correct one and in perfect condition? Also, where did you buy the belt?
Check the condition of the belt tensioner; you may want to replace the tensioner and the idler pulley(s). Not sure how many idler pulleys your engine has. N52 (on my car) has 2 idler pullies.