Mitsubishi Repair: 97 mitsubishi air error code, mass air sensor, valve cylinder head

Question
Hi.
I recently had a head rebuilt on my '97 eclipse spyder 2.0 turbo.  Since it was fixed, the always car idles about 200RPM lower, rougher and when getting on the interstate, at 2800 the check engine light sometimes comes on - the garage says it's an air sensor problem, typical of a vacuum leak.  They've had the car on/off for 9 full days, they say it's not a vacuum leak, not the mass air sensor.  They aren't charging me for the follow-up troubleshooting  :-) .
Other than the valve seals, it's always run smooth and reliably.   Any ideas what to check next?   
* I also tried to find a vacuum leak with ether, but never heard the engine idle change when trying to find it.
Thanks very, very much!

Answer
Dale,
That's nice of the garage not to charge you.  Stick with them :).  Ask how they are checking for the vacuum leak.  A true test is to apply pressured air at the turbo intake, and listen for leaks with the engine off.  When/if a leak is detected in that manner, you slow move up the intake system from the turbo, intercool and piping, blow-off valve, throttle body and elbow, manifold plenum, EGR valve, cylinder head, and then each individual cylinder.  A simple compression test can also reveal problems as well.  Sometimes having poor ignition timing can make the ECU think the there's an intake problem. Hopefully they'll find the cause.
Good luck!