combi 24i keeps loosing water pressure?

my 24i keeps loosing pressure and then the hwc keeps turning off only.
I top up the system and everything works for about 30 minutes then off again.
Checked the value pressure still in the system upto .5 bar??
keep over pressuring the system to get it to run for 2 hours averages 1/2 bar per 30 minutes.
constant water coming out of the overflow.
bled rads and re-presurised already still the same.