[sowise]

Hey Mac,
As far as I know and hopefully someone will correct me if I am wrong but I believe the check valve should allow air into the manifold but not back out from the manifold (I look at it as INtake manifold) so hopefully one of the guru's will verify.

If a dealer tried to charge $400 for plug change I would probably look for a different dealer. Hopefully your stalling issue is corrected.

Most people recommend the the NGK plugs over any of the Bosch. Supposedly the car responds better.

When you say it misses at 4000rpm could you describe it a little more? I wonder if it is a lag in shifting of the tranny or something.

Sounds like your wallet has been getting hit hard over this issue so hoping your luck changes for the better.