I have a customer who has been having problems with severe dimming of their lights and voltage drop on a general lighting and receptacle circuit, when running heavier loads like their vacuum cleaner. I suspected a bad splice or possibly a back stabbed receptacle was the cause. I have opened up every device that I have found on the circuit and moved any wires that were backstabbed to the screw terminals, but the problem persists. I have taken a piece of new Romex and used it as a test piece to replace each section of wire between the receptacles. What I find odd is that when I replace a section with this new wire I am getting a slight increase in the voltage at the end of the line, but I haven't found a section where I see a significant change. Also, if I put a load at the end of the circuit (vacuum) and I test throughout the circuit, the voltage increases incrementally from point to point as I get closer to the panel. I would expect that the location of the bad connection would show up as a larger decrease between those two points. It's almost like there is too much resistance in the wire itself, which just sounds bizarre. Has anyone here run into a similar situation? I'm not sure what to try next.