Basically, If you don't have ReloadOnShow=true, calling show() should not reload the content page.
I see however, that you are using an old version of the control - ClientCallbackFunction was deprecated about an year ago and we've removed it in Q3 2009. This being said, I would like to ask you to open a support ticket and to send me a small sample project where I could examine your exact setup more closely - once I have a better view over your case, I will do my best to provide you with a solution.
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