wow, people take the advice of those who know here- it is not a check valve- get over the whole check valve thing!!! I would check for a leak in the line going to the MAP sensor though- it sounds like a map problem if it runs fine out of boost and then falls on it's face once you hit boost. It could be severely screwed plugs also, but it seems like a map issue though. Even with a bad turbo, the car would just not make boost but still run ok- just weak- since it uses the MAP sensor to "see" what boost/vacuum the engine is under.. If it is running horribly under boost, it is a fuel/spark issue. If the car had a MAF on it and was not a speed density based system, I would say boost leak also- but that does not affect speed density cars in the same way. I would look at the MAP line first then the plugs second.
Joe
PS- If everything "looks" good, break out the service manual or get the specs from someone and test the map sensor with a vom and vacuum pump