Lennox Furnace runs fine and then stops. If we restart it it works for a couple hours and then stops again.?

We have a Lennox furnace that works fine for a couple of hours and then stops. We restart it and it works again for a couple hours and then stops again. We replaced the gas valve and that did nothing. Please help - we are getting up a couple of times throughout the night to restart it and we live in SLC, UT - very cold right now.