that's a good one...all I can think of is put a new themostat in and properly bleed the system...although 100 is not critical in the overheat thing......are you sure the cap is properly secured on the little tank? You might want to make sure of that first, before going ahead with other things.... sounds to me like that is your problem...the overflow/filler tank cap is either not re-installed properly or (unlikely) faulty...just a guess mind you