If it has to do with interpersonal interaction, I think both ENTPs and INTPs can encounter the same sort of boredom that can be interpreted as introversion. The big difference, however, is that INTPs, as far as I can tell, are bored because the interaction is keeping them away from what they really want to be doing/studying/thinking about, while the ENTP is bored because the conversation has developed into a bit of a rut (explaining why the small, two to four person group is the preferred dynamic).
INTPs will remain agitated and drained of energy until they are able to ruminate on what has caught their interest, and this isn't as conducive to social interaction (and thus, Jungian introversion). ENTPs will remain agitated and drained of energy until the situation changes to something novel or more intriguing, by their observation or action, which will spike their energy to a completely outwardly extraverted state. This can be socially conducive, given that there are many shifting variables (such as a party with many small groups that can be bounced among).