Problem with Boxster 986
 

When driving the car it sometimes just cuts out and looses power have had it in Porsche garage and they said it might be ecu might be immobiliser how do I find exactly what it is

Immobilizer, had it been going bad would have some weird alarm behavior as well. Check the car with Durametric or a Porsche diagnostic if at all possible, and see what that turns up. Otherwise, try cleaning mass airflow sensor, checking for vacuum leaks, etc.

If you suspect it's the DME, check out this guide by Pelican Parts that matches up to the symptoms you've been having.

What porsche garage have you had it in? Was this car flooded? Or have you checked under the seat for dampness? That condition doesn't sound safe, I'd guess that it's not the immobilizer because that's usually going to be an on or off thing. Check your wiring to make sure something isn't messed up there, look at the box under the seat. Is it wet under there or any signs of flooding?

Other than that, you won't be able to diagnose this yourself. I'd take it to a dealer or someone with a Porsche PIWIS diagnostic tool. I'd imagine that's the best way to figure that out. Maybe some seasoned mechanics can weigh in.

As you are in the UK, there is a great forum tat is UK based boxa.net.
You should be able to get good local advise there on Porsche shops

Sounds like a bad ground connection to something important like the fuel pump or ECU

Ignition switch? Does the tach immediately go to zero or does it wind down as the car slows?