What are the signs of someone tapping into you Electrical supply?

I moved into my partner's condo unit. It is directly under another unit. I've never met them,but I understand that a mother with two teenage (Goth) kids live there and a building directly across from our unit... The father lives there. The den where my pc is faces the side of that building. Only in one particular electrical outlet does a power surger's light "flicker" nowhere else. I had to put in a new pc motor because the power supply was so weak coming from the outlet. I hear wierd noises within the walls upstairs... I'd like to learn or hire someone if somenone is playing with the wires.