Boiler output is fairly proportional to fan speed so if 3900rpm = 15.1kw then it fires at ~ 13.9kw, on the other hand 2400rpm = 9.3kw which is hardly its minimum output?.
Can you see a settable anticycle time (and (pump) overrun time)?, the circ pump should stay running on any boiler during anticycle, if the recycle time is long enough, say 10 minutes or so then it might be possible to get the flow/return temp down to 25/28C and then just maybe the boiler can get away.
Is there a manual or automatic (ABV) bypass installed?.