Max Tampa switch keeps tripping

Hey all,

I have an older telotyne (sp?) furnace in a triplex that I have a temp setback on. The furnace works fine most of the time but for some reason the max temp safety switch seems to trip when there is a big swing in the outside temp especially if the outside temp goes above zero as it just recently did. To get things working again I just have to hit the reset in the centre of the switch and the furnave will work fine for weeks. I don't think it is related to the setback but I don't know what else it could be. I have a service agreement and they have replaced the safety switch before. Its a frustrating situation as I only know that the switch has tripped when my Tennants complain that the temp has fallen below 68. Any advice would be greatly appreciated.