Pat.
As below mate except make sure that the thermostat is working. are you having to press the red button on the boiler to restart it? what temp is the water getting to compared with the stat setting?
is this a new system? has this always happened?
how high is the tank above the level of the highest rad?
If it continues after you turn down the pump speed you may have to raise the pipe for the vent or even check whether you can swap over to a pressurised system and use an expansion vessel instead, this takes a bit of calculating though. some websites do handle this and you could google the question if you feel the need to go there.
Kenny F
December 2008