partial outlet outage
Since you have used backstabs , time to get better quality receptacles (back and side) and wire them using the screws. 

Backstabbed receptacles are the leading electrical failure in residential wiring. 

The problem is at the last working receptacle or the first non working receptacle. 
With your magic 8 ball tester as they are referred to here a loose connection can show reverse connection when every thing is correct but a bad connection. 

If you have two many wires in a box to be able to remove the receptacle the box fill is probably well beyond code values, I like to use a hair dryer on high when I have intermittent problems it will usually cause the failure and I can check to find the last good receptacle and the problem will be there on the “load” side the side feeding the next receptacle or the line side of the first dead receptacle line is the feed from the previous receptacle.

You can find many examples of backstabs having problems on this site alone, it can be a loose wire nut or broken wire Also but if backstabs are used they will the problem 96% of the time.