Following Ed Beal's suggestion I measured the voltage at each receptacle under no load and with a 13 A load at the farthest receptacle (no. 8). I also checked the drop at the breaker (screw not wire).
Receptacle Volts NL Volts 13 A load
Breaker 123.1 121.9
1 122.3 119.7
2 122.2 119.0
3 122.1 117.3
4 123.5 117.0
5 122.6 116.6
6 123.6 114.2
7 123.6 114.2
8 123.4 114.0
My proposed course is to redo splices at receptacles 3 and 6 and retest.