I posted about this issue a while back, but I really made a mess of the thread and made it very confusing. Reading it now I'm not even sure what I was saying myself Anyway, here it goes:

I do cheap/free IT work for my family's church, and they're having a heck of a problem. They have two buildings, with underground ethernet connecting the two of them. It was thought that they were close enough together to not need fiber (which I wanted them to use), but after everything was set up there didn't seem to be a problem. Now, however, they've been experiencing very slow speeds in the old building (which receives the connection from the new building).

In attempting to troubleshoot it, I took the end of the underground cable and attached my laptop to it. I got great speeds, just what their cable contract was rated for. I then plugged it back into the switch it was connected to, and the speeds plummeted. I tried attaching a different switch and got the same problem. After a little more testing, though, I started noticing that my upload speeds were terrible or the tests weren't even working.

One thing I noticed was that when I connected the end of the long run to the second switch I had, I was getting strange readings from the LED for that connection. It would turn on for one second then off for one second. When it was off it looked like it was blinking nearly too rapidly to see, so I don't know if that means anything.

Is the problem merely that the run is too long? It doesn't seem like it's more than 300', which I know is near the limit. I'm pretty certain that the plans for construction put it at well enough under 300' that they weren't too worried about it. They ran two ethernet cables but both have the exact same problem.

After troubleshooting I set everything back the way it was, but now the connection doesn't work at all. Today I'm going back and I'll try getting them set up temporarily with a WDS connection between the buildings with some routers I picked up. They won't be connecting to the second router wirelessly, so the throughput drop shouldn't be a problem, it just probably won't be ideal.

Is there any way to salvage this wired connection?