Are the solder joints heating up and causing the heatshrink tubing to shrink while the printer is IN USE? Or is this happening during your soldering process, or just due to proximity of the joints to the heaters? If it is occurring while IN USE, then it may be the result of an extremely poor electrical connection and you may need to do a more thorough job of stripping whatever is coating the wires before soldering, or if the wire material is not solderable, switching to a really good crimp type connection. Please take the time to ohm out (measure the resistance of) all the heaters, a comparison might tell you a great deal regarding what is going on.