So, I changed ISPs a couple of days ago and ran into a strange problem. I'm hoping someone here can help.

Previously, I had 40 Mb download and was hitting that mark or a Mb or two higher (Cat5 ran through the walls). After switching to the new ISP, I have a 100Mb download and actually hitting 128-30Mbs wirelessly. However, if I connect through the Cat5 (again, ran inside the wall) I'm only getting 20Mbs. The same is true for my wife's landline. Now, if I connect a Cat5 directly into the modem, I'm back up to 120+Mbs. Also, another landline in the library runs at 120+Mbs. (just tried it again and I'm not getting even worse speeds through this line—2-3Mbs).

Now, it seems obvious the problem is the runs through the walls, especially as they're outside walls and probably dealing with extreme desert heat. But if that is true, then why was the speed cut in half across the Cat5 run in the walls when I switched modems? Is it an issue of too much bandwidth + (possibly) degraded Cat5 cabling = greater resistance?

(Runs are no more than 30 yards).

 

If the cable runs haven't changed, neither has their performance. Most cable suffers degradation at the junctures- what is referred to as Near-end cross-talk [or NEXT]. At the wire ends is where the twisted pairs aren't so twisted.

Point is- it's the plug into the modem jack that may be suffering. Also, it sounds as if the modem itself was replaced, which can drastically affect performance if the OS handles the stack differently.

First thing to try is reseating the cables, and you might consider wiping the ports of the modem clean with an alcohol-soaked swab, just to get residue off the copper.