I have a 7 year old York gas furnace that is acting wonky. It will heat wonderfully, but the unit seems to shut off for hours at a time. Even running it up 5 degrees will not get it to kick on. I have to turn the switch to off and then back to heat. It will either run fine, or it will just off after 10 seconds or so. It is currently 64 in my house (set on 71). Any thoughts?