I originally posted a number of times on the forum with this issue.
For those who have followed the thread, the cause and repair is
now known. The problem was a gasket between the 'A' pipe and
the catalytic converter which leaked as the car was moving and
the engine hit 2300rpm. It only occurred at that rpm.
Thanks to everyone who gave me ideas and direction.
"The only way to have a friend is to be one".
-Ralph Waldo Emerson-