A strange *i/u alternation

What I've always found fascinating about Indo-European is that there seems to be many isoglosses that alternate certain vowels. *e/o alternations in, for example the genitive *-es/-os. Then there's the *i/e alternation in reduplications. Where we find i in Present/Aorist reduplications, and e in perfect reduplication (although we find many variants of the reduplication where the root vowel seems to influence the reduplicated vowel). And last but not least there's the *i/u alternation, which is what I'll talk about today.

I have no idea what may be the solution to this odd alternation, but I thought I'd bring it up to our collective awareness.

Sanskrit has the locative plural -su सु/षु. This is  apparently related to the Greek dative plural ending -σι(ν). As you can see u/i seem to alternate. But this doesn't always happen between Sanskrit and Greek. For example the prefix *dus- seen in both Greek and Sanskrit both display proof for an original *u. Sanskrit has the prefix dus- दुस्- And Greek has δυσ-. For example in Sanskrit you have the word durjanaḥ दुर्जनः 'a bad man' which is quite similar to the greek word: δυσγένεια 'low of birth'.

But then if we look at the Latin equivalent of this prefix we find dis-! It's the dis- we find in dislocation for example. So somehow the *u/i alternation does not seem to be a steady, or split by isoglosses that can be strictly defined in the modern day languages.

I applaud anyone willing to come up with an explanation to this bizarre alternation. I for one, do not have a clue where it came from.