Because interference can be so difficult to track down, we need to take a methodical approach to identify what we are dealing with.
So my first questions are:
What type of ballasts are the MH lights using?
Is it electronic or electromagnetic(more likely)?
Are the lights themselves grounded and shielded properly?
From what I read so far the lights cause issues when they turn on. Once the metal halide lamps are warmed up to full brightness does the interference go away?
I know that electronic ballasts made in china tend to bypass FCC testing and cause problems in commercial applications.
This may be a bit technical for most but here is a good article on the subject:
Grow Light RFI Kit for Digital Ballast To suppress interference to neighbors and nearby electronic devices, we suggest a RFI-BALLAST kit with 1 ring filter on the AC input and another on the ballast lamp output side for each grow light ballast used. (e.g. 20 ballasts = 20 RFI-BALLAST kits...
palomar-engineers.com