Pro-Host Solution (Paypal) return webpage wired issue after the payment

Hi there,
I got website and have a strange issue.  to accept the credit card payment, the Paypal Pro-host solution has been integrated.  

the weird thing is, after customer gets the payment paid and is redirected back to our webpage where the NopContext.Current.User's CustomerID is totally different with the one before customer is directed to Paypal payment webpage.  and it only occurs in Chrome.   IE works fine.  not tested yet on Firefox or Safari.

why the NopContext.Current.User has been changed after redirecting back to our webpage? is it cookie or session or anything else? what is the solution then?

thanks in advance for your advise.

Dave

(after tests, the issue keeps with firefox as well, and in the redirected webpage after payment, sometime NopContext.Current.User is NULL).