combi boiler heating?

i have a heat line solaris 3 boiler it keeps flashing low pressure of between 0.6 and 0.9 bar so i top it up to 1.0 to 1.5 bar then the heating and hot water will work until i turn it off tap or the timer ends on heating but once it cuts off it will flash again and if i top it up again and put the heating back on the pressure then goes to high upto operating pressure of 3.0 to 3.8 bar so i have to release some pressure its driving me crazy im going round in circles