I'd like to second the idea of testing with an extension cord right from the wall outlet, just to eliminate the possibility that there is something wrong in the conduit under the layout.
I do not know if it will have any effect or not, but it sounds like you ran only two conductors through the conduit, leaving the metal tube to carry the earth ground. It is possible that there is some sort of coating on the conduit or that it is made of some cheap alloy, that has a high resistance, and thus doesn't provide a solid ground throughout. I generally see three conductor wire used, even inside conduit with a green/bare copper wire connected inside each and every junction box. Again, just tossing ideas out there.
JGL
