Selhurst Scooby

I have 2010 STI Hatch which has thrown fault code P0172 twice over the weekend. As it's an intermittent error it's very difficult to track down as the MAF seems to be running ok, my local specialist has suggested lambda issues or the dreaded ring land (short block fitted under warranty 3+ years ago and has covered 35k since)
The car starts and drives fine etc but when failing it hesitates and fumes out of the exhaust (black deposits on inside of tail pipe) oil levels are good & I'm not sure it is r******d but pressure test booked in for Wednesday, any similar stories / remedies out there?