"Then calculate the forward price asF = erT * (C ? P) +K, where T is the time to expiration, C and P are the call andput midquotes, and K is the strike at which minimum occurs."

<r>Maybe you can employ the definition of SP500 to determine at-the-money strike: It might be because you are missing the correct strike, so the prices of calls and puts always differ.<URL url="http://www.cboe.com/micro/skew/documents/SKEWwhitepaperjan2011.pdf%22Calculation"><LINK_TEXT text="http://...

How discontinuous is it? I am interested in the problem.