Octavia Tdi limp intermittened mode

Hello everyone. I am a new member but have been reading posts on this site for some years now and found them very helpful in keeping my 2005 Octavia Tdi on the road. Lately though it goes into limp mode and will not respond to the accelerator for a while and the engine light comes on. Then after a while she begins to run perfect again and it could be a few days before she does it again. I checked the diagnostics and the following codes are coming up. 17911 and 16621. I also checked the hoses going to the turbo and did not find any leak. I cleaned the turbo boost sensor (MAP) and I checked the continuity of the wires from this sensor and also from the alternator and their sockets but all seems good. Do any of the good folk here have any ideas?

Regards; John.

Edited 12 January, 201511 yr by germancars