VOKERA 20-80 RS Combi Boiler Problem?

We (in Cork) have an 20-80 RS Flowatic combi which is tempramental (but not half as tempramental as my missus if the heating don't work)... anyway it seems to work OK sometimes and not others.

The pressure seems to drop gradually over time, yet if you increase the water pressure to say 1.5 Bar it will shoot up to over 3 when the things fired up. Sometimes releasing water through the safety valve (I believe)

Sometimes it will come on, heat up the rads and then do little else for the rest of the night and the rads just get colder & colder.

The hot water only works if the heating is on, so in the summer you have to switch the heating on to get a hot bath, or do the dishes etc. It also doesn't seem to stay hot for long fluctuating between hot and luke warm! It has been like that for a number of years, the last engineer told us it would be too expensive to fix.

A couple of years ago the pressure vessel popped on Christmas eve (of course) water through the ceiling etc, so we had a bit of a chilly oul christmas and managed to get someone to replace the part by new years.

Now the missus is starting to get cranky as she's sure the boiler's gonna need replacing and she couldn't bear to have no central heating. (I've got North European blood in me so I'm good down to about +6 degrees)

Any thoughts?