How do i find the source of my wan connection problems (Modem/Line/ISP)

How to i find the source of my internet issues?

Symptoms

sometimes i have trouble with my internet connections falling away. It seems to be on a loop: takes 1 minute to make connection and get an public IP. i have internet for 3-7 seconds and it dies again.

Used to be somestimes for a few minutes or a whole evening, but sometimes when i woke up problem would be solved.

Last time i had this I tried to reboot my router few times without success. I went to bed next morning internet works but download is mega slow. I contact ISP they tell me to restart the router, and afterwards download speed is back to normal.

Same day it starts tripping again, wan drops and restarting it wouldnt fix it. finally i completely resetted it and configured it again and after that it worked again. Then few hours later it died again, this time complete reset didn't work.

Problem seems to have intensified lately, now being really bad.

Possible causes

• Router: TD-W8968 V5 0x00000001 (software: 2.0.0 Build 160905 Rel.56011 tried to update to latest but says zip file corrupt/illegal firmware)
• Router is hacked and used as part of botnet or something. (but why pester me, it would only result in them losing one bot?). Why i think this? I see in the router log 1 intrusion (an attempt of) just before it goes down again.
• ISP: I live in belgium using EDPNET. They make you pay another 100 euro's to use their router but you can also setup your own (which I did). Motive: get us to buy their router. But I would expect people to leave bad ratings of such and i don't seem to find them.
• Line issues: Our house is sort of in backyard of another house and the line runs on a high wall (some big old rose bark growing against it but line doesn't seem damaged).

question

What techniques can I use to rule any of these out? and do the symptoms sound familiar to anyone with expertise in this area?

Edit: added statistics

Note bottom part is a seperate imageNote the page is empty when its re-initializing the WAN, but when its initialized and on (for the 5 seconds) it looks like this.

Edit 2: statistics

Internet finally seem to have stabilized, statistics now look like this:

(Note bottom part is a seperate image)

Currently this is partially solved.

My neighbor (living above me), had the same problems and let somebody come by to check the lines. He found the connection points where not properly protected and thus got oxidized (rusty) and thus the connection got worse.

but one day later we still had the same problems. After that the ISP 'downgraded our profile' basically means they reduce our bandwidth... The internet goes over the phone lines and i'm pretty sure our line is so bad that if all my vertical neighbors use internet the line gets over-saturated and modem can't handle the amount of errors or mishaps i gets or something like this, at least it loses its line state (it goes down).

now with downgraded profile, i haven't got this issue anymore, but that is a terrible (not a) solution. they tell me they can check the lines but the internet lines are technically owned by the house owner so they will send an invoice for that. so i'll have to take it up with my house owner, see if he is willing to pay..