don't discount the nozzle. don't discount anything. Who "cleaned" this boiler last? was a full burner service done? did you see the service get done with your own eyes? if not, don't assume anything was done. Right now it could be the primary control, nozzle, flame sensor, filter(fuel supply), pump, electrodes/ electrode placement, transformer, or burner head in addition to the air band and stack draft settings. you can take a screwdriver and see if the spark will jump the terminals on the transformer with out first making a hard connection - but you might get shocked if you don't know what you're doing.
All these are pretty cheap (not necessarily in this order):
1. change filter, if there's not a filter JUST before the burner add another one JUST before the burner.
2. change nozzle - anything less than 1gph is going to need good filtration
3. make sure overfire draft is set low to avoid flame-stripping from the head (-.02 on a plumb and level baro is a good start with most setups)
4. make sure burner head is clean and intact
5. make sure electrodes aren't burned out and are in good shape
6. check electrode gap and placement in relation to the nozzle (different with different burners, heads, firing rates etc.)
7. many others, but this is a good start.