Hmmm. sounds like a leak.
How often do you fill it? what pressure does it drop to and what pressure do you refill it to? Is yours an old house with old rads/pipes from gravity fed pre-combi days?
Does it drop even if the heating is off?
The overpressure valve won't release until at least 3 bar, so if its the EV you'll see the pressure go up when the heating is on, then it would blow off at 3 bar or so. If the pressure only goes up a little when the heating is hot then it ain't the EV...