re: Electrical wiring issue - outlet reads 120V but not getting full power

You need the total distance from your fuse box to your receptacle in the garage coco

It would be best if you could isolate just that circuit, I'm assuming you have a light in your garage if so I would start with the fuse panel you believe its on and start removing fuses until you the light goes off
There may be other outlets/ switches in line with it so turn everything on

Then remove the panel cover and post up a pic