Natural, why? And how could gas pressure be corrected if low?

Regualtor could be freezing. Would need to be replaced. Gas company normally does such at no charge since it belongs to them. Call your gas company & ask them to check pressure.

Gas pressure checked an OK. Now what?

"And OK. Now what?

Was the furnace acting up when the gas pressure was checked? If not, the test means little.

No and it's not like you just have the gas company suddenly show up during the small time frame that it happens... so now what?

After 30+ years of servicing HVAC equipment I've enountered my share of intermittent problems & they are often tough to diagnose when the customer is around the corner let alone trying to do it online.

You could buy a manometer & measure the gas pressure yourself. Otherwise, I'm out of ideas. I certainly don't want to suggest just starting to change one part after another in hopes of maybe eventually finding an answer.