First, you should read the tdifaq above on loss of power; really.
Here's my opinion:
If your power does not return, say if you are on a hill, when you turn off the engine and then restart, then it likely has something to do with your intake. Your intake could be blocked or it could be your maf. I would examine the egr valve by removing the clamp at the valve, pulling the hose, and looking inside. If it is really choked with black gunk, that could cause your loss of power. If you remove the electrical connector to the MAF, and a good chunk of your power returns, then I would say it is likely your MAF. (Note that the CEL will light. It should eventually go away after it is hooked up again, but I'm not sure when or indeed if it will without getting it cleared.)