Thanks everyone for your suggestions they are all valid and probable cause of the issue. Just to give you an update, last weekend I put in a new MAF (US$100), got a throttle body cleaner and an OEM TB gasket. Removed the the TB, cleaned it up quite nicely (it wasn't that bad). But when I tried to put it back the new gasket did not fit because the SC uses a different one that I had to reuse. I don't know where to look for it online.

So far she runs fine and idles better than before, had to disconnect the battery to reset plate position sensor.

Now I need to find gasket, guess I'll email Magnusson. Also, the TB sensor connector is quite brittle from the engine heat and broke, zip tied it for the time being and have a new one on order.

Regards