> No, the problem is no different whether 'on the bus' or battery power.
This would seem to rule out noise coming in via the power lead (assuming it was disconected at the GPS end of the lead when tested on battery).
If you are having problems when the radio is in rx mode you will probably have serious problems when it's transmitting as the power levels are very much higher - but perhaps that doesn't matter as you transmit less frequently.
I've seen equipment with type approval labels that clearly don't
meet all the standards they are claiming.