Intermittent loss of power, code P.1613?

Hello again,
My Freelander is a manual transmission, M47 Diesel.

The symptoms I’m getting are as follows:

Firstly, after a few hours on the motorway, it lost power with no warning lights on the dash.
Didn’t matter if I dropped a gear and revved it highly, it still slowed down on inclines/barley increased speed on the flat.
When I got off the motorway, I pulled over and switched it off for a few minutes. I then started it again and all power, was restored for the rest of the drive home.
Seems like some Vibrations/Rattling from the rear, in 4th and 5th gear around 40 mph + only whilst accelerating?
Sometimes hesitant acceleration, from a cold start, but in 2nd gear, with a little grey smoke, full power after this?
Sometimes takes longer to start, with a bit of grey smoke clears after a few rev’s though.
Another intermittent symptom has been that when you first start it, sometimes it pulls away normally in first gear but when I change to 2nd gear, the powers gone for a few seconds and it makes an unusual noise, then the power returns and it drives normally.
Since then I’ve taken it to the mechanic, he plugged it in and 1 code came up: P.1613
but he didn’t know what it meant?
He cleared the code and recommended a full tank of decent fuel, also adding a bottle of injector cleaner.
So I filled it up with Esso Supreme Diesel, and added a bottle of “Wynn’s Formula Gold” Diesel system cleaner.

Any ideas, information about the fault code would be much appreciated.
Thank you in advance

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Unplug the MAF sensor see if it drives normal with no power loss, sound to me like the MAF sensor is breaking down, which will put it in the limp mode, and or stop quick acceleration from a standing start at round abouts etc.
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When was the air filter last changed, it may also benefit from having the EGR cleaned and the manifold with MAP sensor which is different to the MAF above.

The MAP sensor is on the side of the manifold and needs removing before cleaning it out.
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searching via google .."p1613 land rover" .. it be a code that has everyone scratching their heads.
'emissions' comes up a lot ..
one lr forum post sais :
"P1613, I've seen this before, I believe it's a cylinder balance correction code."
...................
" Because while the lights vary in appearance from vehicle to vehicle, all have the same basic meaning: There's a problem with the car's emissions system. The on-board diagnostics system and engine control unit are in charge of monitoring a bunch of different, and if they get a reading that's a little out of whack, up pops the check engine light."
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