My RSfourS has both Boost & vacuum to p/s load switch above power-steer pump, 1x straight boost off t-piece near std boost controller, 1x manifold vacuum and goes to never-never!
Have you disconnected battery recently?
IAC idle valve does gunk up over time but ecu compensates, If you've reset ecu then it goes back to factory settings with gunked up IAC valve.
Sometimes takes weeks of normal driving to sort out or quick 5 min ecu-reset/idle-reset procedure, even power fc manual stats it!
disconnect batt - start to operating temp, idle no loads at all, one at a time - low beam then high, a/c on, steering load, wipers, lighter, radio, etc - then all on at once.