Self assigned IP Ethernet, it comes back everytime it's fixed, HELP =)

Hello everyone !

The old self assigned IP problem which everyone is getting. Yup. But this time my dear Mac took it to the extreme :
I've dealt with this problem in the past, where it was just a matter of changing the ethernet ports, or restarting the router, or deleting some file in preferences, the nice tricks everyone shares on the forums and youtube.
But recently the problem reappeared, and has decided to haunt me. In the past days, every single time I managed to bring Ethernet back up by whatever means (first it worked by deleting a file in Preferences, then channels, then restarting the router and finally resetting it), the self assigned IP and the orange light reappeared an hour or so later. And it did so while I was happily using Internet.
Typical situation : I'm happily gaming or watching nonsense on youtube when suddenly, internet "freezes" and nothing will open or work anymore. I immediately open system preferences to check the ethernet tab - which i still lit green and in working order -, only to see it switch to red, then orange a couple of seconds later.

Also, apparently reseting and/or restarting the router have been the only repeatable solutions so far. Changing ports and deleting the file in Preferences only worked once.

The problem defenitely seems to be coming from the router, though no one else in my family is dealing with this issue, and all are on mac! (Granted not everyone uses ethernet, but still)
Even the my brother's ps3 -connected via cable- works wonders for him.

This continuous cycle is going to drive me mad! Worst of all, I don't understand it, or why it is.

Please, any help!
Many thanks, Leo

I'll try your suggestions and let you know, thanks !

In the meantime, I thought it might help mentioning that I use 2 "adapters" (lack of the actual name) to connect ethernet.
What I mean by that is the following :
Router->Ethernet cable plugged into adapter->Adapter plugged into electricity->signal uses electric cables (I believe) to get to 2nd adapter->ethernet cable coming out of 2nd adapter plugged into computer.

It just hit me that that might be important.

I checked the adapters and both them, and the cables, seem to be in a good shape, no traces of damage.

I meant, they haven't experienced it. Granted I'm the only one who uses cable for the mac (otherwise it's used for printer and ps3), i don't recall it happening to any of them ever.

It also never happened on wifi. Yes I have access to wifi but I'm so damn far away from the thing, and that's why I chose the through the wall cable solution, otherwise it's sllooowww.

I believed(ed) to be the router because : I talked to an online-help guy who assured me "99.99% sure it's the router". I agree because every time i restart/reset (most, not every) it fixes the problem, at least for a while, while solutions from the mac haven't brought much.

I personally think it's some kind of weird preference in the router (since there's no physical trace of a deficiency), but then again, resetting it to factory defaults didn't change anything, so i'm clueless.

This morning I got it back up by plugging/unplugging the adaptors, it went down 1hr (more or less) after.
I find it weird, because it is as if the router suddenly decides to ignore and forget my computer.

I don't believe it's low signal strength.
As I mentioned, it suddently shuts down on me while I'm browsing, and I'm browsing at a satisfying speed !
Yes I have Wifi and ethernet on, no idea about the order. I did have both on for a significant amount of time in the past w/o issues.

PS: Ethernet is back up, no clue how/why, for how long, etc...
I was thinking that leases might have something to do with it?

Then again, why wouldn't the router be able to talk to my computer (or vice versa) when renewing the lease?
The lease was and is set to 60min, which roughly corresponds to the ethernet's uptime cycles in the past few days.

UPDATE : Same as before. Keeps going back up and down. I manage to get an IP every time I unplug the router/adapters, then it goes back down again after anywhere from 5min to an entire day...

I think Fish and Blaster Boy are right.
Yeah I've tried manual IP, even everything set on manual and filled it all in, it failed to connect.
I'm indeed using devolo powerline, 200 AVplus. Checked for any firmware updates, and I got "This product is no longer available!" on their site. (go go devolo!...)

The path my cable would have to take to get to my computer would disturb a lot of people, I imagine making it slide closely through the wall, going up and down on door frames, etc... but that would take a considerable amount of time to set up.

I've just downloaded devolo cockpit which should (i hope) help me...do...something? I'll see if it helps

I really wish I wouldn't have to buy a new pair of powerlines, these guys cost me 90€ and seem to be in very good shape (haven't opened them or anything, but they look good from the outside at least)