Jack,

The first thing to check is that the interface is FULLY isolated.
Surprisingly, some commercial interfaces use isolation transformers on
only the RX line. Also, not all use fully isolated keying. Check the
manual, or disconnect all the computer inputs to the interface, and
then check with a meter between the interface case and the rig. Should
NOT read a DC short. (Or probably with the laptop connected without
the mains, check between a metal point on the laptop and the rig.
Haven't tried this, but it should work the same.)

If you find a DC ground, it can be easily fixed with an inexpensive
600 ohm audio transformer. The PTT is a little harder, but not much.



--
Alan
WA4SCA