I can’t help feeling that the periodicity is a clue- at half hour(ish) intervals something
is causing significant packet loss. It could be on Zen (probably is) or it could be something on IDNet which was previously working properly but has been broken by whatever changes on Zen led to all the fuss in the first place.
Either way, surely somebody must know what functions are running at what intervals and check out (or switch off!) the possible culprits?