I have a Rheem RGPH-05 eauer gas heater that was made in 2003. It works fine most of the time but out of the blue it won't start. When this happens if I mess with the thermostat I can hear it clicking like its trying to start. I then go into the attic and bump the unit with my fist and it immediately starts up. This isn't frequent but has happened about 4-5 times over the last 2 years. I discovered the "bump" on accident the first time it happened I went to investigate and when trying to take the cover off I slipped forwarded knocking the cover off and the unit started up. It seems like something is just getting stuck, any ideas what part I show look at replacing?