Simulating a "spark gap" in PSPICE
Maybe you could use a Shockley diode or DIAC model (NOT a Schottky diode).  A Shockley diode turns on when the voltage across it reaches its breakdown voltage and doesn't turn off until the current though it falls below a minimum holding current level.  

Here's a site that goes through creating a SPICE model: http://www.beigebag.com/case_shockley.htm