mk2 1.6tdci 90ps - f34 occasionally blows

Problem:

On occasions the f34 fuse will blow, typically when coming to a stop and idling, e.g., breaking down and stopping at an intersection or stopping in the driveway.  Besides that, I don't seem to be able to spot any similarities when it happens.

Side problem:

I'm not sure if this is related, so I'll mention it just in case.  The car is currently having some issues with the "mixture", which I'm blaming/attributing to a bad MAF sensor for.  When it starts it idles a bit rough and produces quite a bit of white smoke (smels like unburnt diesel).  It sometimes knocks during acceleration, and during acceleration it almost always produces quite a bit of white, gray and then lots of black smoke.  When it knocks (2-3 times) it typically also produces gray smoke.  When keeping a steady speed, it doesn't seem to produce any smoke.

Besides this, it pulls normally, though the acceleration seems a bit delayed, which could be because the MAF isn't responding quick enough or even correctly.
Also it doesn't use oil or water. and doesn't have any logged DTC (well a puddle light, but that not related).

Background:

All of this started with a broken fuel pressure sensor, which was first replaced with a faulty one (***** that took quite some time to figure out) and later replaced by a certified BOSH garage with a working one. 

During my debugging of the faulty pressure sensor I had all the injectors tested by a BOSH garage, and they were within specs, though one of them were only barely within (but hey the car has done +450.000km and I don't know if the have been replaced).

Possible cause?

According to the wiring schematics i found here on the site: 

Then the f34 fuse (page 36) goes through C90#12 and from there only to the "FUEL PUMP PRESSURE HIGH" and then straight to the ECM module (page 37).  

I assume that this "FUEL PUMP PRESSURE HIGH" is the fuel pump pressure regulator control valve located on the back of the pump (like this one)
However the owners handbook says that f34 is for the injectors.  But according to the wiring diagram, then the injectors are driven directly from the ECM, without any fuses.

I have tried replacing the pressure regulator with another one that ought to work, and it still seems to be blowing fuses on occasions.  

To exclude any chances that something was making the valve getting stuck at end positions or something, I tried cleaning both of the valves in the sonic cleaner, but it still blows fuses once in a while.

If the schematics are correct, and if it is not the pressure regulator, then I don't see any other problems than the ECM module/PCM being the issue.   Which I really don't hope.

I'm a bit at a loss at this moment, unless the fuse actually also is used for the injectors, or something else.

I was originally thinking that the issues with smoke was either the MAF or the pressure regulator.  But since it didn't make any difference when i tried the other regulator, then I'm quite sure that it is not the issue, and thus attributing that directly to a bad MAF.

Does anyone have any suggestions?