i had a similar problem today, peaked at 23psi and dropped back to 12psi.
all my boost presets peaked at their set levels and dropped back to 12psi, was very odd.
however a boost hose then popped off and then put it back and tightened them all back up and it then held much better dropping 2 psi, but needed work on map and boost controller settings need tidying a bit.
a cracked housing can cause surge, but it would have to be a big crack to allow that sort of boost drop.