2005 Ford Taurus Turn Signal flashing quickly

My left turn signal on my 2005 Ford Taurus SE will intermitently begin flashing really fast. Both the front and rear bulbs flash, but extremely fast. When I turn on the headlights then that messes it all up. My left turn signal indicator (on the inside) will stay on all the time (again when the headlights are on). And the left turn signal will not function right with the headlights on. I have tried replacing the bulbs many times. I have taken the front and rear housings out (on the left side) many times. First I just put bulb grease inside the connections of the bulb socket and the quick disconnect wiring harness plugs. This seemed to fix it for a while, but then it came back. Second I removed the wiring harness plugs completely (soldered the wires to those male/female plugs that you get at auto parts places). This fixed it again for quite a while. Third I tried unplugging and filling with bulb grease a plug under the trunk, this fixed it for a few days. Fourth I tried spraying WD-40 down into the emergency flashers and cycling them on/off several times, this made no difference (the emergency flasher operate correctly)