potterton 24 performa cycling?

i have followed the answers having a similar problem, the hot water works fine having replaced (some time ago) the diafram now on central heating when the system reaches tempreture, after thie point the system comes on for a few seconds then comes on, this continues for as long as the heating is on. i have removed the inner door panel to clean, i tried boiler (with windows open) and it works correctly, but as soon as the pannel is re fitted it returns to cycling.?? any ideas pls