The switchover valve is on the firewall. It gets a branch of the line from the intake manifold to the ALDA as well as I think two vacuum lines.

It would 100% point you in a productive direction if you just removed the ALDA and see if it still does that. You don't even have to take it all the way out, just zip tie it out of the way once you unscrew it. If it still does it then you can forget the ALDA and switchover valve. If not then it's something in that system.

-Rog