It sounds as if you have your details field set to automatically start a knowledge search each time you type a punctuation character. So although it might appear that it has hung what it is actually doing is starting a new search of your knowledge base with all the text in your details field each time the underscore is pressed. I wouldn't really recommend using punctuation characters to fire off searches on a large or free text field like details as so many could be typed in the process of typing out the field. Instead you could use "lose focus" which means that you only start the search upon tabbing off or clicking off the field. Depending on how you work, you might prefer to switch off the auto search and instead highlight the parts of the text that you want to search on and click on the search icon on the toolbar instead. It all depends on whether your details section is likely to just contain useful text for knowledge searching or also include lots of side information about the customer - so is up to your business requirements really.
Depending on which version you use, the way to change the knowledge setting for details is slightly different but in essence you should be able to go into window manager or window designer, highlight the details attribute on the window and then select properties. You should find that you have "invoke search on" "punctuation character" set to true currently?