Bathroom remodeling may be irrelevant or a coincidence.
Some things I would check:
1. electrodes this is the main cause of misfire. Maybe they are too far or too close to the nozzle.
2. You should buy pressure gauge and vacuum gauge and you should measure oil pressure during operation. You will find a box for oil serviceman on "the biggest auction service" with all hoses and everything you need.
"oil burner pressure"
"oil burner gauges"
3. Check the photo resistor whether it operates properly.
4. maybe air/fuel mixture is not proper. Search auctions for "combustion analyzer" maybe you find something cheap.
5. It can be caused by any factor... you have to eliminate one by one.
Believe me it is not so hard.
In general you have to download PDF manual for your burner and you should read it.