One other thought struck me through all this.
have you pulled the rubber connectors at the switch? My car had oil travelling back up the vacuum line from the shutoff vavle that failed and had filled the lines with oil up to and through the key switch. There is the chance that the vacuum line/switch might be partially blocked. You could still draw vacuum but it might not be sufficient flow to pull the shutoff valve quickly hence giving you a slow shutoff.
I'm fishing now...
