Why don't you get a u-tube manometer ( or make one ) and see if your theory is correct? You caould measure at the bleed nipple on your gas meter, or at an approprate point on your boiler ( see manual ), if you feel you are competent. It may require a bit of patience, but if it turns out to be a drop in the gas pressure, you could examine the gas pipe run from your meter to your combi and see if it could be beefed up. IIRC, it should be 22mm pipe to within a metre of the combi, and even then I suppose it depends on the power output of the combi ( though 28kW should be OK for
22mm pipe generally ). How long is your gas run? How many elbow joints are there?Andy.