Are you running a dedicated vac manifold off a large port, such as the brake booster nipple? I've seen many people with problems because they tee'd in to the FPR line for both BOV and WG. I've seen a boost gauge flutter (hundred times a second) from 0psi to 20psi on a 10psi spring because of that.
The only other thing I can think of is you have a partial leak. It could be a weak coupler that seals only to 14psi, opens at higher boost, bleeds & reseals at low boost. That would do the whole spike, drop, reboost thing. Get a pressure tester rigged up with $5 in pvc fittings (google it) and a valve stem, then put 20psi through it. Remember to put buttonhead screws in the pvc fittings in the spots you'd have a beadroll, otherwise after 15psi they WILL blow off.