Sludged/Furred up HE?

Went to a job today where the boiler keeps 'pulsing' and kettling from inside and the pipework is actually rocking back and forth!, its not anything to do with the pump, as its new and the system has been powerflushed (the old pump was completly blocked) but the problem persists.
So if a boiler kettles can this be down to a furred\sludged up HE and if so why does the water boil if the sludge isnt allowing as much heat to conduct through to the water as normal, would'nt the pump be taking the heat away as well?

is this to hard for you guys ?

Probably kettling because the water isn't passing through the heat exchanger as quickly because of the sludge and is therefore boiling/kettling as it gets too hot.