I have a boiler system. When it kicks on, the pressure rises to slightly over 30 psi and then discharges a some water through the overflow pipe. Not much, I would say about 5 gallons over two days of running in mid-winter. I have changed the pressure relief valve (a 30 psi), so know that is not the problem.

Any suggestions?

Your water pressure or temp in the boiler is to high.
OR your expansion tank is full.

Yes. I agree. The pressure should never go over 15 psi. Check the temperature first... if it is over 200 turn the aqua-stat down to 180 then see what happens. If the pressure still goes up, shut off the furnace. Close any valves in the expansion line and take the expansion tank off. You will know is it is full. It will be fairly heavy. Empty they only weigh a pound or two at the most,, full of water... well a gallon of water weighs about 8 lbs... There is also a relief valve at the top of the expansion tank, that should be replaced at the same time.