Help!

Have a Goodman Gas Furnace, downflow. It works sporadically (and only for a short time, generally 5 consecutive cycles). 45 out of 50 times the thermostat will trigger it to start, I will hear the motor start up (or what I assume is the draft inducer) and then after a minute will shut off. After three times it will automatically shut off and quit trying. Sometimes (but rarely)I will hear the draft iducer motor start up and a sound like the gas kicking on, but then it shuts off quickly resulting in the blower forcing out cold air instead of hot. Pulled out the flame sensor but it didn't look corroded to me, maybe just a bit dull looking (as opposed to shiney). Air filter is fine. Squirrel cage has been cleaned to the best of my ability- no dust or debris. Any suggestions?