pump does not stay running?? potterton prima b boiler

[gazza99]

Any help? potterton prima b boiler. noticed the last few days that downstairs radiators cold when heating set to full. room stat on max. water also not as hot as usual. tank in airing cupboard upstairs with pump next to it. boiler in the garage. have check power supply to pump and this is okay pump works fine and must circulate water as all good upstairs.(radiators)
pump does not run continuousley any more. it used too. it now seems to just cycle on and off approx 1 min on 2 min off.
boiler runs then cuts out. i suspect a overheat thermostat or something but cannot find any info anywhere.
when system used to start on a morning with the radiators only, the boiler made a noise like a kettle boiling until the pump kicked in. this has now stopped but again do not know why.
have changed 3 way valve earlier in the year to fix hot water and heating concern and that worked fine. this other fault has just appeared.
any help would be apprieciated.

 

[Jazzynash]

Does the pump have speed control if it has then turn onto max (normally Number 3) if all rads working fine upstairs normally it's the pump may have to change it or remove and clean..

 

[gazza99]

Does the pump have speed control if it has then turn onto max (normally Number 3) if all rads working fine upstairs normally it's the pump may have to change it or remove and clean..

Pump is set at speed 3 and has not being touched for at least five years.
Do you know what controls pump? is it the boiler, could it be a faulty thermostat inside boiler??
sorry i did not mention that sometimes it all works as it should!!