1930 Model A. Runs great for about 15 minutes

[JDD_0485]

Hi. New to Model A's. Engine has new coil, points, recently added thermostat, no bubbles in water at any rpm, machined head, new gasket, & rebuilt carburetor. Will run fine, great power, after about 15 to 20 minutes it backfires once, sputters, loses power. Pulling choke keeps it going for a while, can be restarted then immediately dies each time. No adjustments help. Coolant never gets above 170. Ideas? Thank you all in advance.