Atomica gives as the source for frenetic:

Middle English frenetik, from Old French frenetique, from Latin phrenēticus, from Greek phrenītikos, from phrenītis, brain disease, from phrēn, mind.

I see the connection with phrenology now.

Hey, does anyone know what governs whether or not words with ultimately Greek origins keep their ph- at the front? Is frenetic spelled with an f because it came through French? (No, wait, they use ph too, as in phoque.) Anyone?