Intermittent p1556 code

The 02 a few times has gone into limp mode for no apparent reason ie: it has occurred at low speed just in town , not horsing it at all.
It has not thrown a code till today but each time it also has "fixed" itself just by shutting down, parking, retuning later and restarting.
I searched for this code and got lots of info but none that had these specific symptoms.
Any feedback?

none better than our low power thread or the Ross Tech wiki

17964/P1556 - Charge Pressure Control: Negative Deviation

Possible Symptoms

• Reduced power output
• Limp mode

Possible Causes

• Boost Pressure too Low
• Hoses/Pipes incorrect connected, disconnected or leaking
• Charger Pressure Control defective
  • VNT (variable nozzle turbo): nozzles stuck
• Solenoid Valve for Boost Pressure Control (N75) defective

Possible Solutions

• Check Hoses/Pipes to/between Components
• Check Solenoid Valve for Boost Pressure Control (N75)
• Check / Clean / Replace Charge Pressure Control
  • Check / Clean mechanism for variable nozzles