get a outlet tester like this: http://www.musiciansfriend.com/pro-audio/american-recorder-technologies-ground-fault-outlet-receptacle-tester?src=3WWRWXGP
If you have a harbor freight close by, they sell it much cheaper $2-3.
Sound like you have a loose ground or return wire somewhere. Sometime, they (electricians) connect return and ground wires to another outlet. Newer outlets are made of plastic and will cracked over time and poorly terminated wires will came loose.
The tester will give you an idea as to what wrong with your outlet.