It aint a problem with cold solder joints. I believe the problem comes from the design of the wire connection going onto a V2 endbell. I asume it must be the close proximity to the brush. I have had mine come off many times, I have lost the wire on both the positive, and negative sides, but not both at same time. When the connection fails the motor brush tube seems to be totaly absent of solder on the tab for the wire (yes much like a cold joint) but, there was so much resistance that the connection just failed. Does anyone remember the TEKIN solder drop fuse. It acts much the same way. When there is too much curent, and or a posible problem the solder acts like a fuse. I have yet to try the heatsink to see if this would help cure this issue, but I believe it will.
As for the weight of this part, you can't be serious that 5.8 grams will slow your car. Thats less than 1/4 ounce. Now just add to this that the balance of most cars weight is off, (the batteries weigh much more than most of the newest crop of smaller speedcontrollers, recievers, and servos) and rarely does anyone go through everything it takes to corner weigh a vehicle and place the weight in the correct areas.