AARRGH! Radio cuts out since installing HEI

[StoningtonQB]

I've been having a problem with my radio ever since I converted to a GM HEI distributor. The radio will cut in and out. When the truck is off, I can turn the key to ACC and it works fine. The green wire for the tach runs thru the firewall but it's a good distance from the radio. I've tied up all of the radio leads and have experimented with some temporary sheets of aluminum foil to separate the tach lead. Everything is decently far, at least 6+ inches, and it doesn't look like re-routing this wire is going to change things.

Nothing seems to be working. The foil was just a test and I'm probably going to take that stuff out of there. I'm running cut-to-fit MSD StreetFire wires which claim "high EMI suppression" so I don't think that's the problem. Is there a way I can shield the head unit somehow from all interference? It's a RetroSound Hermosa if that makes any difference.