First, look into rebates from RI on a new system. Second I think I know a few of your causes. My guess is its not air in your system its gelling. Air would be a constant issue and you would spot a leak most likely. The tank in a cold spot? There is a treatment for that. Also if maintained a boiler lasts a really long time if its a good brand and cast. A friend just gave up using his residential 1894 Steam system that was originally coal because his parents died. On cost, I think the Propane would be cheaper with a condensing boiler but make sure you try to get the rebates.