Ignitor is intermittent

Will come on....gas valve opens....cylcles great
ignitor off....does not come back on but motor is running.
Cleaned all vents....very nasty
tested all components,fuse, thermostat,coils,ignitor,ignitor sensor everything checked ok. replaced coils,ignitor,ignitor sensor and dryer dried one load now dryer is doing it's cycle mentioned above again. I guess my next step and the only one I can think of is the timer? If so how do you check? Or the gas valve? I didn't check voltage to coils.....

Thanks....
e31430