Hi, I have a potterton supremer boiler and a gledhill boilermate 2. It seems to occaisonally make a gurgling/bubbling noise. It goes when I either turn the thermostat up to winter setting then back down again or when i run the hot water tap for a while. It seems like it's overheating the water or something. I resently had the circuit board changed on the potterton and it worked fine at first but now this. Any ideas?