Nissan Primera P12 dci sudden turbo drop out / in

For the last about 40.000km the turbo pressure on this car has gradually dropped. During the last 10.000km I've replaced turbo boost sensor and maf, fixed the valve controlling the vacuum to the turbo boost controller solenoid, cleaned egr, driven with an active style with egr disconnected, and finally reconnected the egr and run a tank with diesel additive. By doing all this, the turbo pressure has doubled from it's worst point. I think it's still a bit left to the original level, but it's good enough for my use - until today.

Today I drove the car 550km on various roads, including highways, sharp corners and steep hills.  The first 480km went fine. Then all of a sudden the engine lost all power. Turbo pressure was just 20% of what use to be after the mentioned repairs. That calls for the need of using 3rd gear on the highway in tiny uphills! This problem lasted for about 20km, then the problem disappeared just as sudden as it showed up. Then after about 10km more the problem came back - and it became gradually worse until the point when turbo pressure was -0.3psi (that's a negative number). Intake pressure was 100 kPa independent of engine rpm (that's what it usually is when engine is not running). Engine refused to go past 2400rpm. And MAF reading topped out on 17 at 2400rpm (Normally about 50-60.) At this state I had to use 1st gear to get up the hills where I normally use 5th gear.

This lasted until I was 5km from home. Then it got into the 20% pressure state again. And 500m before home it suddenly jumped back to normal again.

There is no fault codes logged. No check engine light.
I can hear the turbo spinning when the turbo pressure is in 20% state.

I consider this problem as something that will reoccur. Then most likely far from home at the most inconvenient time. What could be the cause of this problem?
My suspects:
1) Turbo turbine broken - but then it shouldn't fix itself by driving.
2) Something blocking air intake. But the intake is clear. Also the air filter is fine. It was new 4000km ago.
3) Unstable electric connection to the valve that is controlling the turbo boost control valve solenoid. I've had such issue before, but the car didn't get as weak as this.
4) A breakcalipper stuck/slow retracting. But that shouldn't affect the turbo pressure reading. The ECU readings should be more like steep uphills even going down. Also, break problem should affect coasting. My coasting is not affected.
5) Leaky air inlet tube. How to check?
6) EGR stuck fully open. It shouldn't become as bad as this, should it?

What else should I check / rule out?

Headlamp cover is where it should be.
This is a 6 speed manual transmission. Does grounding to the transmission affect performance on a manual?

I did not switch off the engine along the way. I was thinking switching off would either solve the problem or make it not restart at all. I didn't want to take the chance with a baby in the car. I didn't expect the problem to fix it self 500m before home. When I got home I restarted and drove some additional miles, but the problem did not reoccur.

Yes, it does make sense that it can be a earth-related issue. As the current state is ok, I'll have to wobble the connections and hope to find one that is loose. Are there any places, except for the rear light, where earth tend to break?

Are there any electrical components that possible can block air inlet in case of electrical faults?
Could this all be down to a sensor failing? Maybe if a failing sensor tell the ecu there is just a little bit of air flowing into the engine, the ecu will cut back on fuel supply, hence the engine will run too lean. I've experienced the opposite - MAF reading too high, causing ecu to push more fuel and engine will smoke a lot.

With the issue yesterday, I could not see any smoke in the mirrors. Not even in the lights of the cars passing.

I've had faulty MAF and turbo boost sensor before. Neither of them threw error. They provided signal, but the value were 50% too high. I've now placed the old sensors in the trunk, so when the problem reoccur, I'll swap sensors to see if anything changes.

I've looked for loose cables (including ground) today, without finding any potential problems.

Yes. EGR looked clean. It was operating fairly smooth when engine was cold. It could have been stuck with warm engine. I gave it some carb cleaner and a few runs of wd40. Now it operates perfectly smooth with cold engine. If it was stuck, I see it could explain the 20% turbo pressure, but could it explain the 0 turbo pressure?