I'm having difficulty trouble-shooting a Coleman Evcon furnace with a Robertshaw HS780 ignition control module. Problem is intermittent, first time I arrived to customer's home furnace was running fine. Cycled unit several times but could not duplicate problem. Unit has in-shot burner with HSI. Second time I went out t-stat was calling for heat, ventor motor was running, had 117V from control to ignitor and 117v back to control, but no voltage (24V) from control to valve.Re-set power,ventor motor came on, ignitor came on, but still no voltage to valve, unit locked out again, reset power again this time unit lit up and ran through a complete cycle???? I'm suspecting faulty control but want to eliminate weak ignitor/sensor. My question is how do you correctly check these units for flame sense? is it in series with the ground terminal on the control. I haven't run across many of these units over the years so any advice would be appreciated.Customer has had ignitor changed 3 times in the past 2 years for same type of problem. If it is indeed an ignitor problem why aren't they lasting longer??