heating not working properly

[alhendy]

Before I call somebody out, was looking for info please. Boiler works fine on heating water, but only warms up 2 or 3 of 4 rads upstairs and doesnt heat 3 downstairs at all. Have tried balancing but no avail. On display on boiler (vailant) flame symbol comes on for max 14 seconds, temp display goes quickly upto about 81 degrees, then symbol goes out, temp drops to about 50 degrees, taking 3-4mins. Then heats again for about 15 secs and so on.
Blockage, air, sludge, room stat...advice gratefully accepted before I call someone out

 

[lame plumber]

need a gsr I'm afraid

 

[Dotty]

Look at the reply I gave you the first time you posted this question..