Simulating a discharge circuit

Hi,

I am trying to simulate a discharge circuit, using this circuit:
R3 ( 1 2 ) 0.01
R1 ( 0 1 ) 2e6
R2 ( Vhigh 0 ) exp(iv=1e9, pv=3u, td1=1u, tau1=9n, td2=1)
S1 ( 4 3 4 3 ) swmod
Vcc ( Vhigh 0 ) 25K
C1 ( Vhigh 1 ) 1.7n
T1 ( 2 0 3 4 ) Z0=50 TD=37.7n
 .model swmod sw(von=5K, voff=-100K, ron=5)

[I run a transient analysis from 0 to 3e-6s with a 1e-9s step and output 
the current through the test resistance R3 which is where our current 
probe is situated in our experiment]

The voltage controlled switch represents the electrical breakdown above 
a voltage of 5KV. This circuit works quite well in reproducing what we 
see experimentally. However, there are artifacts caused by the fact that 
the switch changes immediately from high to low resistance.

So, I would like to know if it's possible to have a voltage controlled 
switch as above but which has a time-dependent model for its switching?

Thanks,
Tom