problem with potterton performa 30 he cutting out to quickly

[scorpsmyster]

I am experiencing a problem with my potterton boiler performa 30 he when in use and travelling up the thermostat bar on boiler it gets to 80 degrees the burner stops it then takes 3 minutes to travel back down the bars to 50 degrees at which point burner fires up again it takes about 40 seconds to reach 80 degrees and 5 seconds later burner stops again and the whole process is repeated again this is with thermostat on boiler at max when put at half way the same happens but only fires up for 35 seconds and as soon as hits 80 it shuts straight off again. i have monitored it coming on and off 10 times each on both thermo settings with same results, off for 2:55 on for 45 secs high temp, and lower temp off for 2:55 on for 35 secs. any suggestions or is this what it is supposed to do? boiler is 10 years old in last 2 years i have had expansion vessel changed and pump and some other o-rings on another part but cant remember what that was. any help appreciated as my boiler insurance keep telling me that's normal

 

[Boilerman2]

In the Right hand isolation valve under the boiler, (heating return)
you will see a nut facing forward, AFTER ISOLATING ELECTRIC & WATER
SUPPLIES, drain the boiler, remove the nut, there is a mesh filter (strainer) inside, my guess is it is filled with crud, wash it out, and replace , refill and put back into use, problem solved

If the system is very dirty, consider getting a powerflush carried out