Did you ever do a compression test? This is highly unlikely but here’s my theory: Bad compression (when cold) till it finally lights off, and then once it warms up everything expands to the point that compression gets high enough to run well enough to start regularly.
Just getting back to the very basics: Internal combustion motors need fuel, spark, and compression.
I know it’s not likely, but anyone think my theory has a snowball’s chance in hell?