I have honestly had this problem. After running an inverter in the socket for a few minutes, it stopped working. Checked the fuses and all were good. After about 20 minutes with a multimeter I found that the wires were still live and the problem was the socket. If they are put under heavy load the socket itself will give out. It is the ground wire prong on the back of the socket that loses the connection with the casing of the socket. I fixed mine by soldering a wire between the ground prong on the back of the socket to the side of the socket casing.
You can easily tell if you have the same problem by setting a multimeter to OHMS, touch one lead to any exposed chassis metal. Touch the other to the cylinder of the socket. If you don't read a low OHM value then the socket needs replacing.
I was literally in the same position as you, problems since I owned it. Once I soldered that 1/2" piece of wire I haven't had problems, though I should have just replaced it.