Hey folks. 

  I've been racking my brain about an odd problem that I've been encountering.  Hope you SW wizards can help. 

  I'm building an assembly from weldment parts, rectangle tube to be specific.  About half way through the assembly, I ran into this: when I define a mate, all seems well.  The mates are accepted, and the part fully-defined in the correct location.  When I click the green check, the part moves away from the mated location randomly for no known reason.  Here's the kicker - if I click the mate icon again, the part returns to its mated location....until the mate utility is closed again.

  I've tried everything.  I've restarted SW, tried to add different parts, all that jazz.  Played around with fix's and floats (I usually fix everything once mated since my assemblies don't branch from a common parent part).  I've tried locking the mate, it still moves after I exit out.  Nothing seems to remedy the problem.  I'm sure my mates are defined properly, as I build similar assemblies daily.  I've never run into anything like this, and I'm about at my wit's end. 

Many thanks in advance for any help y'all can provide. 

SW 2013 Pro