Default Loosing cellular signal randomly

Running CM11 nightlies, phone randomly drops cellular signal when on global. If I switch to GSM only, it appears to be more stable, but still drops occasionally. Phone will be stationary, have 2-3 bars then suddenly be in a deadzone. If I try to make a call or open "OpenSignal" app, service is restored.

I have a similar issue where GPS does the same thing.

WiFi and BlueTooth appear to be unaffected.

I've experienced this on 2 Atrix HD handsets, so while hardware issues are a possibility, it seems unlikely.

Any help would be greatly appreciated.