gas boiler problems?

I have a glow worm 38cxi and it is about 5 years old it has been serviced yearly but it has always fluttered and made a loud noise when the hot water tap is only on half way,this will dissappear if you turn the tap on full but you do not always want the water at full speed as it splashes everywhere.also in the last couple of months the boiler seems to constantly refill itself slowly with water and air,nearly every day I bleed one of my rads as it squeeks everytime the heating starts up,plus I have to release the water pressure about once a week.