IF your autofill is out of the picture, and the pressure rises when the boilers are running, your expansion tank is probably shot. Check it. It should be precharged to your normal system pressure. This must be done either before you install, or if in place, with all water pressure removed and a drain valve open, otherwise, you'll just be reading the water pressure. The gauges on the boilers may not be accurate - when operating, you can use a good tire pressure gauge on the expansion tank to check it...it will read the same as the water pressure unless your precharge was excessive. If the system was regularly getting high, that puts more stress on the expansion tank, and it is probably bad.
You want enough pressure in the system to prevent boiling or zero pressure issues. Take the rise from the boiler outlet in feet to the high point and divide by 2.31. That would be your minimum pressure - I'd probably add a couple of pounds to that. If it is more than half of the pressure relief valve, you may want to switch it up IF the system is designed for it to give you a little working room. In a typical 2-story house, 12-14# is enough, and a 30# relief valve works. You may want one higher.