My '97 Boxster had an intermittent no start problem. I suspected the DME and/or immobilizer and had the local Porsche dealer verify it with their Porsche diagnostic equipment ($300). They said 98% sure it was DME/Immobilizer, but they could not fix it. Instead of having them find an outside vendor to fix it I took the car back (total of $225 towing). I searched for ECU repair on Google and found two places, one with horrible reviews, the other (Specialized ECU Repair) with good reviews and recommendations on the Porsche forums.
I sent Specialized ECU Repair my DME, Immobilizer, and key. Their fee is $149 for diagnosis. It turned out the Immobilizer needed repair $696, and they shipped it back $29. I received it yesterday, installed the DME and Immobilizer, and the car started immediately. So far no problems.
I recommend this company.