My home ac\heater mounted on the roof

Normally when activated I hear a low hum for a few seconds
Then I can hear the flame go on
A few seconds after this the fan goes on.

Recently the flame goes on and off until eventually it stays on

Then it works normal after that.

Sometimes the flame will go on and off for a long time before it
Works right.

Not sure if the flame is always on once the fan starts blowing
But it does warm up the house eventually

What part do I need to change and would you have it available

Not an expert here, and my furnace is different than yours.

Does the fan also stop? As in fan and flame kick on, then flame kicks off and fan stops? Repeat. Repeat...

In my case my flame sensor was dirty and just needed cleaning to keep it from cycling over and over. No clue why it sometimes seemed to sense the flame and work fine, and sometimes not. Light cleaning with sandcloth did the job...

Now ill step aside for the real expert advice. We've got some good help here, so if I'm wrong, at least I bumped your post to the top of the new threads.