Shutdown doesn't always shut down

i have mid-2011 27" imac i5 base model. on several occassions, when select "shut down" apple menu, re-start instead of shutdown. before i'm selecting wrong item on menu, have done testing i'm 100-percent positive i'm selecting right thing, doesn't shut down, re-starts. doesn't happen time, enough annoying. there times when want shut down, can re-boot in win 7 under bootcamp, have shut down second time after imac re-starts instead of shutting down. has else experienced problem?

 

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