The extension cord would not be a problem unless it felt warm in your hand after running the pot for some time. (The warmth would be from too small wire in the cord acting as a heater, which would cause a voltage drop. But still not likely to lower your pot temp.)
The lines in your bullets, sounds like a drop of lead which solidified before the cavity filled completely, which is caused from the mold being too cold, and can be partly from pouring sideways as the stream begins. When the mold is hot enough, the whole thing will be full before any solidification begins, and will not produce wrinkles or lines like you are getting.
One more thing. Lee pot thermostats are in the box at the back, which if cooled by a breeze, of if the pot is used in cold weather, will not let the thermostat cycle when it should and the lead will overheat. I've seen them run the lead red hot! In hot still air, it will cycle way too soon and not bring the melt up to proper temperature.
The least expensive lead thermometer in the world is a pine stick. A wood kitchen match stick is fine. Dip it in the melt for a few seconds. If it comes out well browned the lead is hot enough to cast well. If it comes out black, or starts smoking quickly after inserting into the melt, you are running quite a bit hotter than necessary. If at night or in a shop with no windows, turn the lights off and look at the melt. If you can see a very dull red, temperature is at least 850 deg F, 900 to 1000 will be a fairly bright red.
