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In some words, the stress is on the first syllable of inter, for stance, intercourse, interview, internet, interval.
However, there are also some words, in which the stress is on the second syllable of inter, for instance, interpret, internal.
What's the cause of this difference?
Internet is a compound formed according to English compound rules from inter- and net(work). When compounds are formed in English, each part retains its stress and the stronger stress goes on the first part. So internet is pronounced with the same stress pattern as a compound noun like "underwear".
Although intercourse and interview did not originate as English compounds, I think it's plausible that they have become interpreted as such, based on the fact that course and view do exist in English as independent words.
In contrast, interval, interpret, internal end in -val, -pret and -nal which have no existence as independent words.
Why is there a difference in stress between those three? I'm not sure.
Internal follows a common pattern for adjectives ending in -al: they are stressed on the second-to-last syllable if there are two or more consonant letters between the al and the preceding vowel letter. I discuss this rule further in an answer I wrote to the question ""Substantival" - doesn't fit the normal stress paradigm" posted by mbm29414 on ELL.
Interpret is a verb, and its pronunciation might be related to a principle that Greg Lee notes in his answer to the question Why do nouns and verbs which are stressed differently all exhibit the same variation?:
Nouns and adjectives tolerate more unstressed syllables than verbs do.
However, interview is also used as a verb and has the same stress in that case as when it is used as a noun.
In the case of internal, it's because it uses the Latin prefix in- as opposed to ex- (out of). The other words are based on inter- meaning between.
This doesn't explain interpret, though, since that also carries the sense 'between [two people]'.
