What is a FW DI unit?

I've been testing connections and joints between my guitars that don't make noise (or a lot less noise) and the ones that do.  I'm finding that my solder joints -although silver, shiny and pretty - are not as consistent as the factory soldered guitars.  Digital equipment is a lot more sensitive to this, I'd imagine.  My solder joints should instantly read without having to move the probe around.  Though the tops of my joints don't indicate cold solder, it may very well be that the blobs underneath didn't heat up all the way and what I ended up with is a cold sandwich on the leads.  The same hold for the hot leads.  This is the only explanation left for me.

It's clear that I won't be able to troubleshoot without having to unsolder everything and clean off all excess solder and then resolder.  I'll do this on one guitar - the simplest - and see if it yields good results - ie:  no/less hum.

What do you guys think?