I've never been clear on the origin of the alternation seen in the Latin verb <premo> / <premere> / <pressi> / <pressus>. The IEW's position (IEW pp.818-19), if I understand it correctly, is that <prem-> and <pres-> show two different (and semantically opaque) derivational suffixes attached to the same stem, but I don't know of a single other Latin verb that shows this kind of contrast within a single paradigm. Are there any other theories on the origin of the <premere> / <pressus> alternation?
My best guess so far is that the <-m-> of <premo> / <premere> is a nasal infix, assimilated to a labial under the influence of the initial <p->, though that doesn't fully explain what the original stem would have been (<pressi> could reflect <*pred->, <*pret->, <*pres->, or perhaps simply <*pre->). Do you see any problems with this theory?