system switching off?

getting hot water but no heating had a new pump fitted still no heating ,then we drained the system refilled it and switched on,all the rads heated up but once it switched off at the wall stat the rads star ted to cool down ,each time it fired up it would switch off after 20 seconds or so. so we bought an addative to clean the system which we put in through the refill loop.after 3 days we then flushed the system again and fired up, but same again lovely and hot at first but once the rads got hot it switched off again . please can anyone help