furnace short cycles. it ignites for several minutes than cuts off than restarts again & again

if it is a newer unit...it could be as easy as cleaning the burner sensor. they build up with contaminates. turn off the unit with the thermostat, making sure fan switch is off and heat/cool is in off position. open main cover of unit and find the sensor rod that is in path of the furnace flame. there are usually 2 such sensors, one with 3 wires and the other with 2 wires connected to it. you want the one with 2 wires. disconnect the 2 wires from the sensor rod and CAREFULLY remove it. don't bend or treat it roughly. then using a nylon 'scotch brite' pad or nylon dish scrubber, clean the residue off the rod. DO NOT use steel wool!!! after smoothing up the rod and getting the contaminates cleaned off the rod, re-install rod and wires. re-mount main cover and switch the system back on.