It's hard to diagnose your problem - other than ruthless driving - without seeing the car in the flesh. First time I got a 4WD buggy after I started racing (2wd) was a XXX4, and I drove it like such a nutter that I couldn't last 3 minutes. full power, full brakes, full power, full brakes etc... No car is going to survive that so lets rule that out
I use some weller radio solder from b&q so I don't think solder is your problem - if your wires are getting that hot then it's not right. I don't have the air holes on my Durga cut out and don't use a heat sink etc etc.
You've not taken the motor apart have you and possibly reassembled it with too much advanced timing? loads of advance on the motor would do this.