Hi there, new to the board and hope you guys can give a few pointers. I have an 11 year old Ultra SX 90. The unit will start up without issue, run for 20-25 minutes and then it shuts down before reaching the desired temperature as set on my Honeywell thermostat. After about 5-8 minutes it will come back on and finish heating to the set temp. Start up is normal with the blower first, and the blower continues after it shuts down for about 60 seconds or so. The filter was changed a week ago. So for any minimal increase in temperature required, or to maintain, the furnace works fine. When a significant increase is required (mornings) it wont stay lit for one continuous heating blast. Thoughts?