Coleman furnace flame quits after 20 minutes
It sounds like a component is failing. Could be the circuit board or the gas valve. When you say it tries to light does it glow and fail to ignite? Does it ignite briefly and then go out?  Most likely a component is getting hot and loosing connection. Could be the vent proving switch if there is no glow. The combustion air is being taken from the roof since it is a sealed combustion furnace. The return air is coming from inside the house. 
The vent motor should come on. Several seconds later the igniter should glow red. After a few seconds the gas valve should open. 45 seconds or so later the blower should come on. By using a volt meter at each component check if there is 24 volts present at each component at the proper time. If there is voltage at the gas valve for instance and the valve does not open it is the gas valve. 
I once worked on a furnace for 2 years before it would fail for me. The owner said it would not operate when cold but by the time I got there it was working fine. Finally it wasn’t working when I got there. I was getting voltage to the gas valve but no gas. There was a bad solder joint inside the valve that would make enough contact once it warmed up a bit.