problem with inverter in proteus

When I try to simulation the following design in proteus, the software always shows that timestep too small,trouble with node....(the node about the switches).When i delete the switches circuit,the simulation can work well. Help me ,please!

what type of swiches do you use?

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you can upload the DSN file let see whats goin on.

new.rar
Thank you for your reply. The attach is the DSN and HEX files.

Dear kabeer , I have solved the problem,and I changed the switches which are for about 50V, the new switches are for 500V.