I wouldn't be bothered with the tester because it take more time to get to the store and buy it than fixing the damned thing.
It is likely that one of the leg on the series is broken or loose between [box 1] and [box 2], box 1 is the first receptacle on the series that get power. Open the face plate of box 1 and box 2 and tighten the the terminal screws and plug a light in to see if it work. Replace the receptacle if there is visual damage/burn marks on the receptacle, and may need to strip the wire back to fresh copper or add a new pigtail.
Power Off at panel before commencing the work.
==========[box 1]==========[box 2]==========[box 3]
[edit]
I didn't read all posts, but it seems that you have 2 receptacles on a series, and the other is on another line with the room switch.
The theory above still stay true, that the loose or broken leg is between box 1 & box 2.