Line thickness is not the issue, at least on NVIDIA hardware. I'll be happy to solve this mystery as soon as MARS_999 provides a reproducer. :)

Solution has been found and reason why, thanks to Gold.

Reason is
It’s an interaction between user clip planes and polygon mode.


Solutions are

You have a few options to work around this.
1) Use a vertex program instead of fixed function. You’ll have to implement your own user clipping, as that is part of fixed function. We don’t fall back in this case.

2) Use lines instead of triangles in line mode

3) Disable user clipping in wireframe mode

Hello Mars, excuse me to bring this thread from the grave but I'm having a major performance drop while rendering in GL_Line mode. I try to find information on how to disable the user clipping and I couldn't find anything so far... so my question is, how do you disable the user clipping?