I have a Heil furnace (10 years old) 100,000BTU. Problem is thermostat closes, inducer blower kicks on, ignitor lights then goes off, lights then goes off, on and on. This may go on for as much as 30 minutes before burner ignites and heat is supplied to house. Pressure switch is about 5 years old (new switch never seemed to solve problem) and .65 WCPF.
Now I live at an elevation of 10000 above sea level.
I have been told I need new furnace(would be good idea to go to 90% eff from current 80%) or maybe a high altitude pressure switch (haven't found one yet)
Any ideas out there?

Clean the flame sensor if so equipped. Look at the back of the blower door for a diagram and possible location. EASY does it. Steel wool,fine sand paper work well but the sensor assembly can be damaged. Also stay away from the ignitor. That is the item that glows to light the gas. It is really sensitive to body oils on your fingers and is very fragile.

Well, I did take the ignitor out yesterday (prior to reading your e mail) and noticed that the heat sensor (thermocoupler?) seamed a little far from the ignitor (1/2"). without touching the ignitor I bent the heat sensor about an 1/8" closer to the ignitor. Replaced it and all seems to be working fine.

Glad you got it going.