Before people start suggesting some expensive fixes I suggest you try changing the pressure cap on the coolant expansion tank. If the cap isn't holding the pressure the coolant will boil at a lower temperature. Whilst under pressure the coolant can take a higher temperature before it starts to boil. You can guarantee most garages will check the pressure in the system but hardly ever check the cap which holds the pressure in the system. Its a consumable part which should be changed regularly anyway. Good luck with it.