Blower Switch Overheats

All speeds of my blower ( heat/cooling ) fan work fine...but after a while the blower switch on my dash gets hotter than hell. This happened last year to the extent that the heat 'melted' the switch control knob. I replaced the switch and thing worked ok - no overheating. Recently, with the summer heat and need for air conditioning, the damn switch 'froze' on the high setting. That is, it physically would not move off the high setting. I removed it and took it apart to find that the internal metal contacts were destroyed. I then replaced the switch and the switch/fan seemed to go ok for a while, Very recently, with the onsite of temperature in Chicago, and with continued use of air conditioning, the switch started to get very hot again. I thought it might be the blower resistor. I removed it and it looks and appears to be working fine.

Any ideas????