Try putting some sewers aid on the thread. I often have dry/weak spots in Coats & Clark thread and the sewers aid really seems to help. Also try a new needle--1 size larger.
You don't mention which model Janome 1600 you have. Some of those take the round shank needles so it might need to be turned just a hair.
It is also possible that when your needle broke, it put a bur on the bobbin case or knocked your machine out of time.