Randomly losing power while driving (long post - sorry)

Hi, I'm not sure if I'm posting this in the right place, however I'm having a rather intermittent issue with my Nissan Almera.. it's an 03 Hatchback, 1.5L engine, Petrol. It's had front driver side suspension, lambda sensor and timing chain replaced in January 2015, then in November 2016 it had a new exhaust (not the whole thing, it rusted at a bend just before it connects to a box, so nothing else needed doing) and at the time the garage said the muffler was on it's way, however it didn't need immediate attention and warned I might hear rattling, I might not. I promise this might seem relevant in a sec..

Back in December 2016 my partner was driving along the M11 (coming back from Gosport, going to Suffolk) and pressed on the accelerator to maintain speed of 70(ish)mph and the car didn't respond. The car began to slow down, he tried to accelerate again, but the revs went down and the car slowed significantly, as if he was pressing the brakes (I honestly thought that's what he might have done...). He swapped down to 4th gear and the car still didn't speed up or anything. We ended up having to pull over as far as we could and the car just stalled. No warning lights or anything came up to indicate battery/engine/oil problems. After 5 minutes, we tried again but just a few miles up the road, the car did it all again. This time we went into a layby, the car stalled again and we checked everything we could - power steering, oil, coolant etc. There was absolutely nothing we could see that would cause the problem.
We gave the car about half hour and set off again, quite nervous and started thinking that maybe someone had used some sort of EMP (as a joke or something). I dismissed the idea and said noone else was affected, and why our car..?
Anyway, we made it back home and driving locally (shopping, work etc) it was perfectly fine, nothing out of the ordinary at all.
It happened again on the way back from Sussex, on the M11 near Stansted airport in late August 2016.
It did it again on the way back from Gosport in December 2016, but this time on the M25 just after going through the Dartford Tunnel.
It then did it again on Monday this week on my way home from picking up my partner from work. I was only doing 30(ish)mph and it had already driven roughly 23 miles. I didn't turn the car off when picking my partner up, so I'm not sure if maybe that's an indication of anything.

Note: Going down to Littlehampton or Gosport the car never had any issues - we even joked it didn't want to go home. On Monday morning, the car again didn't have issues driving the normal 25 mile work trip.

The battery is ok, I had this tested last night at Halfords and the guy said it's in perfect health and when describing the issue, he said he couldn't pinpoint what the issue could be, as it could be one in a million things.
I've currently got the car in for an exhaust check to make sure there is nothing wrong there as well as a diagnostics test to see if they can maybe identify what could be wrong..

From this rather long(sorry) decription, is there any clue as to what the issue could be..?
 
Update: The garage has been unable to find anything wrong with the car at all as they couldn't replicate the fault during a test drive, they've checked the car's computer and found no fault codes on there, they've checked the exhaust and there's a slight deterioration of something (can't remember the specific wording) with it but nothing that would affect the performance of the car.. the best they suggest is: take it to a dealer and see what they say.
What disturbs me is the fact the car just randomly stops working, there's no previous warning, there's no warning lights to at least offer some sort of idea of what's going on
I was kinda hoping they'd come back and say "We found this, it will cost £XXX and we'd need it for a few hours.." but... it's maddening.. and I'm now starting to panic in case it just stops and just won't start - which is when they'l finally figure out whats wrong and then I'll get moaned at for not trying to get it fixed sooner! (I've had that discussion before)

Could it be the engine starting to fail? (I'm not a mechanic, I don't know if a car's engine will 'start to fail' or if it would just go noooope and just not work..)
 
First thought would be the timing chain, but seeing as that has been replaced I would say the MAF (air flow meter) or possibly one of the sensors that monitor the timing chain if they weren't replaced when that work was done. Odd there are no codes though.

Problem is, you don't want to start throwing parts and money at it to see if the issue goes away.

Either way it does seem electrical if its an intermittent issue that goes away when restarting the engine.
 
I know its an old post but I had the same problem which cost a lot to find and annoying because it couldn’t be replicated by the garage.Finally got a loan of a Central Processing Unit and swipped it out and that sorted the problem forever.A dangerous issue if you’re accelerating down a ramp onto a motorway and you’re speed goes way down instead of up
 
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