Warren, I have a PSR-600, and have run it occasionally as mobile and I've not had any issues with overload/desense/intermod with a VHF-hi whip. And I live very near a major telecomm site (Oat Mountain) in the LA Basin.
However, when used as a base installation with an external antenna, it used to get intermod interference until I put about 3db of attenuation by splitting the antenna feed- since then it's been fine. I had the same issue with my GRE-made Radio Shack Pro-2055's as well. They are just a little "too sensitive" if you live in an RF-heavy area.
The issue I have is I dont have room to permanently mount a full radio chassis in the car. Thus a need for a control head.
-Rob