Boiler cycling

I have just replaced my old oil fired boiler with a Ravenheat (screwfix) condensing gas fired one, and replumbed the system for fully pumped, vented, using the existing 11 radiators. The problem appears to be not enough flow through the boiler. After the sytem has warmed up, but before reaching room temp., the boiler flow pipe temp. rises to thermostat cutoff. It then falls to the switch-on point after about 1 minute, and relights the burner for 1 minute before cutting off again. It does this continuously until the room stat is finally satisfied.
If the pump speed is increased to position 3, it pushes hot water UP the vent and cold feed and into the loft expansion tank.(??)
I don't think there is any air in the system
and there shouldn't be a blockage as I've flushed out the system several times now.
Anyone got any ideas PLEASE????.

I am not a plumber / heating engineer, but acording to
http://www.technosolution.co.uk/diy/centralheating/CHSystems/systems.htm

The 5th bullet down says
"Above the cistern you’ll see a pipe which hangs over it. This is called the vent pipe and acts as a safety measure so that excess pressure can be relieved. "

so if you turn your pump up (and the expansion tank gets filled with hot water) does that not imply that there is a blockage? (i know you said you checked but the symptoms indicate otherwise)

only an idea.





[Edited by breezer on 05-01-2003 at 15:54 PM GMT]

Thanks for your reply.
Yes I'm beginning to think there is a blockage or restriction somewhere, but where? - its mostly new pipework. As it pushes water up the vent, I would think the restriction is between the vent and the pump input, which is only a matter of inches.