You stated that you have a screentip on a RadRibbonButton? Could you provide the XAML of how you did this? In the meantime, this link will bring you to the documentation on how to properly use the screentip.
Can you make sure that all computers on which the issue is reproduced, are using the controls with version 2012.2.607.1050? Because in the previous releases we had an issue with the ScreenTips opening 20seconds after hovering over the button, but it should be fixed now.
Also, if you manage to isolate the issue in a sample solution referencing the Q2 2012 release of our controls, you can send it over so that we can investigate the case further.