pressure relief valve opens

I have a gas boiler with steam radiators. My boiler pressure keeps going up to 5 psi and the pressure relief valve opens. I adjusted the pressuretrol (0.5 psi with differential 2 psi) but cannot solve the problem. The burners seem to stay on even though the pressure in the boiler has passed the cut out setting. I am wondering whether this happens due to pressure relief valve, pressuretrol, or other sensing components. Recently some erosions in returning pipes have been fixed by breaking a joint and replacing smaller size pipe (from 2 inches to 1 1/4 inches) in some part.

Thanks.

The pipe and or inlet to the pressure-trol is plugged up with crap.

If that isn't the problem then the pressure-trol itself is bad (though very rarely) or someone has screwed up the wiring because regardless of thermostat and or aquastat settings the pressure-trol is the main operating limit for a steam boiler. Thank god the pressure relief valve is working because if it had failed to open you are looking at a catastrophic explosion.