I have had a problem with this in the past. Try shading the product without edges. This will be the very top selection. If the lines don't return to the nornal color then I can't help you. If it does correct the problem, then it seems to be a graphic problem. The problem only shows up once in a while for me. You may want to try to reset the graphic settings within the product or parts.
Slaen - this is going to sound like a Dell support call, but have you tried to save and exit Catia. Restart Catia and load the assembly again. Every few hours you should exit the software, you will get more predictable results.
Slaen - sounds like you have some unresolved constaints. What constraint type did you use to create this unresolved constraint? What are some of the constraint types you have on the part before you created the new unresolved constraints.
Slaen, This is a shot in the dark but did you add color to your parts from within the product structure? I know that on some versions of CATIA this will turn constraints, planes and axis systems black.
