Thanks for the response!

Why does the amp make it worse? This is a campus setup. We have the access point + antenna and amplifier in a central location. The amplifier provides the needed boost to get the signal out to faculty residences on campus.

Here's another data point:

Sitting in a close room in a building near the edge of signal coverage I get a stable 30-35 SNR, but if I move one room back I get an unstable signal jumping between 0-25 SNR (and rarely in between). I still get a working connection in the back room, but I'm definately loosing packets frequently (ping -t gives 2ms for about 5-15 times, then drops a packet).

The fact that I can get a stable 30-35 SNR in one room makes me agree that it is likely interference rather than a cabling issue. Additionally, as I said, even if I remove the antenna/amplifier, I get weird SNR jumps in the same locations.

The idea that reflections of the original signal is causing interference makes some sense, but I wouldn't know how to track that down and minimize it.

What is the best way to pinpoint items causing interference? A spectrum analyzer? Hiring a wireless company to come out?

Thanks.