2002 Subaru WRX won't pass inspection because of check engine light?

I'm trying to buy a 2002 Subaru WRX off of a guy but he refuses to sell it to me until he can get an inspection sticker on it but it won't pass inspection because the check engine light is on and when they plug it into the computer it puts out a misfire code. His mechanic has been working on the car for weeks now and has been consulting with other mechanic's and supposedly even contacted Subaru about the issue but they just can't seem to find the problem. They have already installed new spark plugs, a new mass air flow sensor, a new timing belt, and have cleaned out the fuel injectors but the check engine light continues to come back on. Every time they finish fixing something they reset the ECU but after 30 or so miles the light comes right back on. Anybody out there have any idea of what the problem could be? I have read forums all over the internet but have found nothing pertaining specifically to my problem. Can anyone help?! =(