Our furnace is running longer than usual, and then is off for a short amount of time and comes back on, most of the time. Sometimes it does not come on for quite a while, though it sounds like it is trying to come on. The blower doesn't turn when it is not coming on, but the gas is for a short time and then shuts off. We would like to change out the blower assembly, but can't find one anywhere. HELP?