I hope I manage to describe and explain the problem clearly.
English is not my native language so wording may be off.

This started when I was driving home one day, I was coming towards a intersection and I pushed the brakes, when I came almost to an stop the the car some how instant breake twice (similar as if you were on a bycicle and some one showed a stick in the wheels) and when I went over the lights the car was kinda powerless, but shortly gained the power again, when I was home I heard some strange noise, but wasn't sure, I thought it was just sound in the snow getting pressed down under the wheels, then I took off again and I clearly heard that something was wrong and stopped.
I called the local MB mechanic and he arrived with some extra oil for the transmission, he checked the stick, almost no oil and the oil was rather dark, but no burn smell.
he put some more oil into the transmission, the sound was a little lower.
he did some checking and thought it was safe to drive short distances, to the store and etc. Until I could bring the car in for repairs. The following days the sound became less and less and was only noticeable when the car was in park or neutral, and somewhat when I was backing up, could not be heard while driving or stopped while in drive.
There were ideas that this had something to do with some rear drive pin or few things that would be noticeable when they would take the transmission down (I dont know the English words).

They took down the transmission and noticed some scraps of metal in the oil pan. So they decided to dig deeper, and found some damage to the star gear as we call it.
it was replaced and everything put to gather again, oil replaced ofcourse and some O rings and etc.
the sound was gone and driving was good, I drove it for 5 minutes to the store and on my way back from it, the car started behaving strangely.
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It became kinda powerless even if I gave it more gas, so I decided to floor it so the car would go down a gear, and that fixed it, the repairshop had no idea what it could be and found nothing wrong with it. The car drove fine for days after this. But the sound came again slowly again.
After few days the car became powerless again, I floored it so it would go to to lower gear and gained power again, just like last time.
And again they had no idea what on earth could be wrong, but we're absolutely sure it wasn't the transmission.
few more days went by and everything was OK.

I had to go to the capital city which is about 400km drive, no problems there, except when I was driving up the snowy mountains and the 4Matic would kick in (the yellow triangle) the car would some how swing its behind alittle, like the rear wheels weren't getting the same torque.
I drove around the city on my second day with out any trouble, but on my third day it started acting up again, getting powerless many times, but in short sessions, maybe for one second and then everything was OK again for few minutes.
later that day it started acting up again, getting powerless and would not fix it self, so I floored it like I've done before but with no luck, I saw the RPM rev up but not gaining any speed, instead I lost speed but could hear the engine trying to increase the speed. So I stopped flooring it and drove it 2-300 meters to where I could park it and check it out.

Everything seemed normal, enough oil on the transmission and looked and smelled fine.
But I did notice that when I would put the car in 2nd gear it would make some kinda purring (kinda like a cat makes) sound as I let go of the gas pedal, did not get the sound in either drive or 3rd gear, only in second gear.
decided to drive back home but taking it easy, no sudden speed increments or driving to fast, took the grandpa stile.
I was able to drive home with out any problems at 80kmh.

So has any one any idea what on earth is going on with my car?

 

You have the 722.3 4 spd auto transmission on this w124
It's well known for it's issues, mostly the reverse bands.
It's also known for it's eco crap wiring in the main harness, in the tba and the lower harness.
Have these been swapped out?

 

No not to my knowledge, now that you mention the wiring, I bet that it's located somewhere where it can get wet if its raining, right?

 

All that I mention are under the hood.
It's not necessarily a wa ter issue, the eco-wiring insulation is soy- based and breaks down.
Google it.
If your getting any symptoms of misfires, erratic rpms, acceleration, pretty much any engine, tranny issues, with this wiring still in place, diagnosing and resolving any of your problems
is off the table. The harness has to be replaced with an updated one first. As well the throttle body internal wiring,
Check the tags and label on both of these, first. The lower harness is not so much an issue and can be rebuilt (but you need to keep all the connectors, as most of them are nla)

 

Alright thanks for that khomer2, I'll take a better look into those wirings.

 

Read here

example, when you carefully open up, to inspect.

 

Edit:
*Wrong*

 

Update:
The car doesn't have the ECO wiring.
The strange power loss was the ignition hammer, it's been replaced and all is well now. 7-9-13 knock on wood