Furnace cycyes fails start diferent seqence at diferent times somtimes works corectly. note: fan comes on,igniter glows red, gas valve opens buners start, blower does not start burners shut off cycle starts over. and sometimes somtimes fan on, igniter glows red, burners dont start cycles over again. not sure what part to replace due to diferent seqence of events. Light on control box is blinking constant so all ok that I see?