I've built an application that logs in to a couple of different instances of Salesforce.  For each instance, it does the typical login and setting of the SessionId into my binding object.

My trouble is that suddenly with one of my instances (a Professional Edition instance), every once in a while I'm getting:
Invalid Session ID found in SessionHeader: Session timed out

It doesn't happen every time, so I'm having a devil of a time tracking this down.

Anyone know have a suggestion as to why one might see that error sporadically?

Naturally I've triple-checked the credentials used by the application.  "Lock IP Addresses" is off and the default session timeout is set to 8 hours.