This thread is getting a little long, so I thought I'd bring it back to basics. Plugs and wiring get hot because of resistance, or as the word implies, difficulty in the electricity to get past a point, and cause heat. Best example of resistance is the element in a toaster, high resistance, more than anywhere else in the circuit, and it gets hot. These high resistance places can and normally do eventually fail, like a corroded blade on the plug, or corrosion in the gripper blades inside the power pole, or where the wire connects to the blade in the plug. Yes, small gauge wiring can get hot, but it sounds like the basic wiring is OK, your plugs are failing, so there's where the problem is.
Couple of things. When the copper gets hot it anneals, that is to say it changes and gets brittle=higher resistance. When you replace the plug, do you cut back to where the copper is bright and shines like a new penny? Try not to touch the copper with sweaty fingers (salt) same for the plug components. Buy some no-ox (radio shack has some) but be sure it's not the stuff that insulates (different stuff for different application).
Tighten the screws but not to the point where they begin to strip, and after a few days, tighten again. Lastly, as mentioned above, put a little no-ox on the blades of the plug as you connect. Might not help, but sure won't hurt.
johncmr