Igniter lights, burner ignites but burner only stays on momentarily.
Other times burner may stay lit for several minutes, and yet on other occasions: no igniter, no burner. Attempted to remove igniter to check continuity and it fell apart in my hand, replaced with new igniter. Igniter works fine now, but still sporadic operation. Checked continuity on 3 thermostats , all were ok. How can I determine if its cycling at the proper temperature? My gut says high limit...but what do I know.
Thanks for your help.

Thanks a lot for the info. In my previous post, I forgot to mention that I had checked the flame detector continuity as okay. Is it possible that flame detector could be bad even with a good continuity reading? It makes sense to start with the cheapest first, so I just ordered the coils. I'll let you know how I make out.