mosfet pspice
I will try to help, but I am not sure it will be adequate:
You are using the MOST as a switch? If so, in which voltage does it operate? Did you tried transmission gate instead of single n-MOS or single p-MOS?
Have you placed dummy ( MOST connected as diodes) before and after the switch?
If you do the above, your switch will work perfectly, it will be well isolated from the rest of the circuit and you will see no distortion.
Is it a VLSI circuit or a PCB circuit? Are you modelling the DC Chopper or you are creating one out of real components?
D.