[R-390] warm outlet?

[Dan Merz]

Hi,  not exactly 390a but related.  What is most common reason for a
wall outlet getting warm when drawing about 8 to 10 amps?  This is in
relatively modern (70's) home with 3-wire  wiring and otherwise behaving
itself.  I'm asking this on behalf of my daughter who reported this
happening at her home about 150 miles away and has stopped using the
outlet for the heater associated with the behavior.  I'd have a look at
the outlet innards if I were there and told her she probably needed to
replace the outlet. I'm fairly sure it's copper wiring as she mentioned
seeing copper wires when replacing a light fixture earlier this year.
Any other suggestions or similar experience?  thanks,  Dan.