When you get the reduced performance bong of death and no fault showing , question is is it always the auxiliary coolant pump ? Mine has just done it a couple of times but my vin number would indicate I'm outside the wiring issue .
Thanks in advance . 2008 MY Supercharged
Rimini Red / Jet
four zone climate
remote park heater
and no ugly kid windows.
magnus satis quod turpis satis

Are all your ECU's up to date ? The older software on some ECUs (engine management specifically) may have too tight limits for older models and see values go outside these limits, resulting in this sort of behaviour. That's all I can think of now. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

When mine did this it was because the aux pump had failed. The wiring was correct just the pump had given up. You can test it by wiring a 12v live to it and see if it spins. Quite a simple and cheap fix if that's the problem.

Thanks guys , software is all up to date afaik Eugene and I am leaning towards a straight forward failed aux pump , just a shame it doesn't throw a code . It only does it during a long run I stop for something and the set off hard say a long uphill . 2008 MY Supercharged
Rimini Red / Jet
four zone climate
remote park heater
and no ugly kid windows.
magnus satis quod turpis satis

The pump might be failing / may have failed indeed. No faultcode could be right as it is a minor component maybe or the motor is still measuring OK (i.e. not burnt out/shorted) but not doing enough or the reduced performance is the "fault code" for these types of failures ???

Anyway, I hope you sort it out soon !!! MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!