Oil Fired CH advise

Hi all,
I've been doing a lot of online searching and am after some advise Re; our oil fired central heating.

Symptoms started when we heard a noise from a downstairs rad that was like an air line near the TRV, even when the heating and hot water was off. The CH was still working ok at this point though. (the only time this stops is when the mains to the pump is switched off)

Then, a day or two later we noticed a noise like a tank filling so I checked in the loft and the CH header tank ball valve was constantly running. I changed that and the noise stopped. the air noise from the downstairs rad remained. I bled all the radiators but no difference.

Then, we heard a noise from the pipes leading from the boiler up to the pump. It sounded like an inconsistent tapping/banging with the odd rush of water. I went upstairs and bled the pump but no air came out.

Since then, Ive got some more air out of one of the rads upstairs but thats about all the air I can get out, however, the boiler fires up and can run for an hour or so and then it stops, stays off for a while then restarts and fires and stays on for between 4-7 minutes then stops again. It doesn't lock out though.

I've tried a new room stat but it made no difference. I'm stumped now and was wondering if its a pump issue. I checked the 3 port valve and all pipes going into it were warm. The pump wasn't hot, but merely warm and seems to run smoothly. I spoke to the boiler service fella and he suggested it probably wasn't a boiler issue. I would appreciate any advise that anyone could offer.

Boiler is a Boulter Camray Quartet and we have plenty of oil.

Many thanks, Paul