You're not soldering directly to the extruder, you're just soldering the wires onto the pins of the wires going into the connector that goes to the top of the extruder.
Several of us have seen the wires on that connector come loose and this seemed to fix the problem for each of us. Here is a link to the connector where the wires break loose:
http://www.soliforum.com/misc.php?actio … mp;preview
For me, it was not at all obvious this was the problem. The wire was making intermittent contact, so sometimes it looked like it was okay, but when flexed, it never made the connection. I just pulled the pins out of the connector and restripped the wires and soldered them to the pins then pushed the pins back into the connector. I haven't seen this problem recur since I did that.
You should re-adjust the trimpots after resoldering the connection. I found mine were way out of whack once the power pin was reconnected.