Is the water feed off, you jump from 12 cold to 25 hot. I had a new
boiler and expansion tank a few years ago and went from 12 cold to 35 or
so hot, and was leaking the relief also, the installer put in an
expansion tank that was to small, solution was he added a second tank,
now its fine. But I dont know if this could apply to you as I had a
much greater jump in pressure. You only need enough pressure or
altitude to get water to the highest radiators, try lowering it till it
is sorted out, could your gauge be bad reading low