This sounds like "shortcycling" to me - you may not be trucking the heat
away from the boiler quickly enough, so the boiler stat turns off (to stop
it blowing up), but the room stat is still demanding so the pump keeps
running and circulating water.
I had a bad case of this due to oversized (more is better) boiler for me
hovel, but managed by fitting a couple more rads and turning down the
boiler - not a user control, allegedly, to make it pretty good.
In ay event, it's no more than an annoyance, and working as intended for
safety, and if you have a serious power mismatch, may not be curable.
BTW, could you ask outlook express to word wrap to 75 characters or
thereabouts, the end of your lines is outside my door.
Though good old Xnews does rewrap on for me, I don't know if other readers