Originally Posted by Mister Two

theres two water temp sensors on the RS. one is a single pin plug used for the dash cluster. And the other is a two pin plug sensor used to give exact temps directly to the ECU. Just a guess but, maybe your two pin water temp sensor is gone and the ECU is not running properly when cold as there is a separate map for cold starts. combined with the vacuum line issue causes your issues.

Replace the water temp sensor which I believe is screwed in the metal water pipe running under the intake manifold and see how you go.