Don't claim to be a expert on this but have been running a older miller for best part of 20 years in my place and it sounds like the lenses on the flame sensor may be dirty or sooty.
I have had similar problems where my nozzles plugged up and igniter
wasn't getting stream shot past it properly, so got incomplete burn at beginning of cycle, and the soot caused photo cell lens on flame checker to get dirty,kind of a oily sooty coating on it ,making it act really glitchy.
I clean mine with a cue tip dipped in dawn dish soap and water.
Make sure it is dry tho after you are done as electric and water don't mix and if you don't feel confortable using that zippo lighter fluid will work, we use to use it to clean tv tuners for oily dust because it is a self evaporating cleaner.
If that don't work try replacing the reset box it is basicly a breaker like on your house wiring and just like the ones in your house ,pull
to hard on them for to long and they get weak.If this solves the problem then you need to check amperage draw on furnace because something may be pulling to hard.