Boiler Pressure Trouble

[rhp485]

I have a Worcester 24i Combi boiler. It's 7 years old. I use to have to top up the pressure because it dropped to ½ when the system was switched off and went cold. The pressure needle went end stop, 4+, when the heating was on. Now it does not drop below 1 it stops on 1½ when the heating is off but it still goes to 4+ while the heating is on. I have looked for and found no leaks in the radiators etc. The expansion is at the correct pressure and the air pressure switch has recently been replaced. Any idea what the problem is and how I can resolve it?
Thanks

[dedgars]

Check the manometer (pressure gauge). It may be damaged, dirty connecting pipe.
Check the pipe that connects system, it may be clogged too.

[dedgars]

Check the pipe that connects system and expansion tank, it may be clogged too

[deggers316]

its ok for it to be 1.5 but i would have thought the pressure relief valve would have opened on anything past 3 bar..it will climb when the heating or hot water is needed but 4 is excessive

[rhp485]

deggers316 - you are on the right track. Some rusty looking water did come out through the overflow pipe when the pressure value was turned. It wasn't much or fast and it kept stopping then starting again until it just dripped out slowly. Any more thoughts?