Fan Clutch Breaks Loose

I have a 98 Montero 2.4 5-spd. This has happened twice already. I would be driving and for some reason the Fan clutch would come loose and destroy my radiator and the radiator shroud. The radiators for this car is rather expensive. The car has had a front end collision, however, that has been fixed. There is about 1 1/2 inch clearence from the fan blade to the radiator. My mechanic seems to think its the water pump since the fan clutch runs off it. The shaft which attaches the fan clutch to the water pump may be slightly off and when it turns under high speed it may cause it to break loose. I have paid to get this problem fixed twice. Is it some kind of defect? If anyone has some input please reply.