[R-390] GFCI issues

[rbethman]

This house that I'm in, was built in '79.  The GFCIs installed appear to 
be installed sometime afterward.

I've just now started having issues.  While NOT related to an R-390A or 
other such piece of equipment, it is still driving me insane!

I have replaced receptacles, AND have replaced the GFCI receptacle.

The crazy thing is that the "appliance" that is causing this to trip is 
a hair dryer that has it OWN GFCI in the power cord. These never trip, 
yet every time we try to use it, (Even a brand new one right out of the 
package.), it trips even the NEW GFCI.

I'm beginning to wonder if there are issues in the house wiring.

The one thing that REALLY puzzles me, is that a 450Deg F heat gun, does 
NOT trip it.  It happens to be an old one that I used to use for 
covering the airframes of R/C aircraft.

Anyone have any useful ideas or suggestions?

Bob - N0DGN