Solder becomes more brittle every time it is heated and cooled. The answer is yes. It certainly could be the problem. I would have to suggest that you test with just crimped lugs first. I'd be willing to bet that your problem goes away. Then if you want soldered wires do it with all new lugs and new solder. ALSO: This part is extremely important and why most people stay away from soldering. You have to heat the joint before you put the solder in there. I don't mean press the solder on and once it melts you're good. I mean heat the wire and the lug for 20-30 seconds with your torch before you ever touch solder to it. Continue heating while you apply the solder. The reason is, for the solder to flow evenly through the wire and bond to the terminal, they both need to be at or above the solder's melting point. If they aren't (like they are not when you just start forcing solder in as soon as it's melting) you will get a cold solder joint every time. Also if you move the wire/lug before the solder is hard it will be a cold joint every time. It doesn't help that us guys like to "test the joint's strength" as soon as its cool enough to touch. It's just not smart.