Thanks to everyone for the help. It's a 20 amp breaker, not a 30, the wiring
is all copper, and I had already checked everywhere for a loose connection.
So, following Toller's suggestion, I replaced outlet #3, and all is well so
far. After closer inspection and testing, it appears that the other outlets
were actually wired in series downstream from outlet #3. Even though #3
voltage tested normal, it apparently had some internal problem that prevented
the current from getting to the other outlets. Your advice has saved me the
cost of a service call from an electrician. I really appreciate the help.