Missing anything?

If you buy a cheap digital ballast it will not have a good enough built in rf sheild and that is why you will get signal interference you could build a faraday cage around it and it will stop any outgoing frequencies.

Is the interference coming from the ballast itself or the wire?

It has caused a multitude of problems... I am writing from my laptop using my phone as a hotspot atm...

As the cable guy tells me there is a super strong signal (coming into the house is what he says now) at about 25hz, when it should be from -10 to 10hz. He tried all sorts of things but my modem just wont connect!

Been without internet for the last few days for the first time in 10 years. Its an experience I must say... Also I own and operate a tech business - phone is tied to the modem! Driving me nuts!

On the bright side shes looking beautiful =)

Wow that sucks, i bought a well think ballast for my set up which i am pretty sure is about the cheapest ballast you can get, it was about $80 for a 400 watt digital ballast and i have my ballast less than 5 feet from my modem, I've had no problems so far.

Here are some videos on faraday cages.
http://www.youtube.com/results?search_query=diy+faraday+cage&oq=diy+faraday+cage&gs_l=youtube.3...3438.3619.0.3843.2.2.0.0.0.0.58.58.1.1.0...0.0...1ac.1._B5L191Qds0

Another solution is to use a magnetic ballast. No signal interference, but they are not as efficient. It is coming from the actual ballast. The digital ballast operate on a much higher frequency than magnetic and depending on the range can mess with a variety of household items like cordless phones, wifi, bluetooth, microwaves. They all run on the same frequency either 2.4 ghz or 5 ghz.