Simulate a MOSFET as a switch

Hello,

I want to simulate the MOSFET 2N7000 as a switch in the following circuit : http://hpics.li/63a328a

I measure a differential voltage on the load, the important thing is when I close the switch, the both capacitors discharge on the load. The result of the simulation is : http://hpics.li/e4e139a

I have the good curve shape but not the good voltage values, the peak where is the cursor is about -315mV.

Now if I just simulate with switches as here : http://hpics.li/68b9736

the result is on the following link : http://hpics.li/e1184c4

I have a peak of  -700mV on the cursor.

I have a real voltage decrease and I don't know why. Maybe there are parameters to put in the simulation profil or something other.

I don't have a good english langage, so if my explanation are bad, tell me.

Thanks.