I have a Glow Worm Swift Flow 80 that keeps dropping the pressure down to about 0.5 to 0.7 PSi when the heating in not on. When it's on, the pressure can go up as high as 3 Psi. It all seems to work o.k. but could I have a problem. I've bled the radiators and they seem fine so I'm just wondering what else I can do? Any help would be much appreciated.