Customer called because her gas furnace is not maintaining temperature. Problem is intermittent. Found gas furnace locked out on a code. Gas Furnace runs for up to an hour then shuts down and restarts several times on a soft lockout then finally locking out on code 8 (5 faults in heat cycle) with a 1 hour lockout. Checked igniter, flame sensor, gas pressures, draft pressures, temp rise. After running for about 30 minutes, furnace locked out 5 times on pressure switch fault. Recommend replacing the pressure switch, and customer will address the gas pressure problem with his propane supplier. Temperature problem resolved. Customer will purchase preventive maintenance for regular maintenance. Customer will recommend Advanced Heating & Cooling to all of her friends.