How have you got it idling that high? With the idle screw? Or have you adjusted the cable?

Indeed the big pipe running from the rocker cover, the whole engine itself is a potential air leak, so even a leaky rocker gasket or sump gasket is going to give you problems.

Well, you could remove the intercooler and have it pressure tested, or do it yourself with an air line and bung the holes up and see if you can hear leaking air.

If its in a poor state like you say you might of blown a hole in it, standard ones are nasty things Perfect excuse for a nice front mount if you have