The 18V sources you have used are floating, they should be grounded (switch closed)
When open: via R2 and the higher impedance of R8/R9 (1K) the output will influence the centerpoint of your +-18V power.
thus making an unwanted and uncertain feedback.
When you close your switch, the overall amplifiaction becomes too large and you'll get overdrive on the output.
I think that the effect of puttig an electrolytic (100uF) cap over the open switch will have the same effect as the switch.
it will limit the floating effect of the +-18V supply
Johan
