Ok let me explain something....
All turbo's get hot, xtremely hot when in operation so just because we're having hotter weather at the moment 30'C + is not really anything to do with the lack of water molecule content of cooler more densor air for this level of power loss discribed.
If the turbo has a partially seizing VGT mechanism from carbon build up the raised daytime temperatures at the moment could be sufficient enough to cause metal expansion of the turbo and hold the mechanism from moving thats when the turbo has almost zero power from start up in the afternoons.
Staring the engine cold from over night the turbo mechanism and casting sizes will be relatively smaller inside so will not to be affected as much by carbon trap. This allows the mechanism to move and operated as normal until the mechanism becomes affected by heat soak.
When stuck these mechanisms will probably take a shock or impact on the external control rod to get it moving again, much more than the strength available from vacuum or servo unit controlling it.
If your mechanically minded and have the tools to remove and confidently strip and rebuild the turbo then go for it, but they are not straight forward and patients is required due to access being limited at the back of the engine. Drive shaft could also be an obstruction and may need to be removed. This is a full days worth of lab for a professional mechanic/technician like myself just to give a time scale on work involved.
Dave O
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