How to generate triangle shape?
Don't ask why, but probably it NGspice fails simulating with a pulse width of 0.

The following will work:

V1 0 1 PULSE(-1V 1V 0 {8.335m-0.5n} {8.335m-0.5n} 1n {16.67m+1n} 0)

EDIT
It seems a known issue: https://sourceforge.net/p/ngspice/bugs/355/


  Are you aware of the fact that setting a (pulse)
   parameter to 0 does not mean that it becomes
   0.000000... but that SPICE sees this as a don't
   care and substitutes it with something (what it
   thinks is) reasonable? I would expect your actual
   pulse-width becomes 5 or 10% of the period (I didn't
   consult the source code for the exact number).