our gas oven has issues. it heats up and gas lights. but it never reaches set temp. says preheating for a long time then seems to stay at about 355 if its set for 400. gas goes off and then doesn't come back on although the electric element stays red. when I turn it off and reset it for max, gas will light for awhile then goes out. wife is ready to shoot the oven or me. I've read about potential issues - dsi, gas valve, thermostat, spark unit. can you narrow this down for me??