Mystery solved. Turns out I've been running with intermittent turbo boost for
months. Went to check if the EGR would hold vacuum last night and got a sucking sound. The hose from the firewall to the EGR was rotted almost all the way through. It couldn't be seen as it was on the underside behind the EGR. Cut off the rot and reconnected, and its like a different car. Its good to have an effective turbo.

Thanks to Steve for his excellent explanation, and everyone who too the time to read my 'mystery'.