|I am connected via a self-ran cable to a different house. The line is just under 100m, buried shallow. |
Ok, first thing. 100 m is the maximum segment length using Cat5/5e/6 cable. If you're "just under 100 m" that's going to be an issue. I've found once you exceed about 85 m, you can start to get attenuation which can cause signal degradation.
Has this cable been tested? If not, I'd start by testing it with a proper tester. Not one of those blinking light units either, a real tester. Most any electrical company or network cabling company will have a good tester. Call a couple, get quotes on testing a single cabel (it should be cheap) and have that done so you can see where your cable stands.
The internet works fine down at the house with the modem but on the other end of the line it is spotty.
This to me says the issue is the cable. One thing you can try which may help is putting a SOHO Router at your end of the connection so you're going directly from router to router. I'm going to tell you right now, I don't think this will help. I believe in your case, the cable is just too long. But if you have another router around, or can get your hands on one without spending money, I'd try that for a couple days to see if it helps.
For more info on how to properly add a second router just click on my name above in this response and read my “how-to” guide titled, “Add a second Router to your LAN” You'll want to use the single subnet scenarion wherein you interconnect the two routers "LAN port to LAN port"
You might want to give some serious thought to using wireelss point-to-point antenna's as a solution. I have used EnGenius equipment (if you're interested, google that name) and found them easy to use and quite good. If it were me, I'd go wireless in this instance.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.