Has it had an alternator fitted recently. ? I have seen a couple that kept throwing belts off and it turned out to be the alternator pulley that was out of line, There are several alternators available for these vehicles and the pulleys aren't always interchangeable as they throw it out of alignment. It is very difficult to see if these pulleys are lined up but it is critical especially when the belt comes off and whips the timing belt off.

H.T.H. Tony.