Sudden, brief episodes unsteadiness - like for moment world starts spinning. v. disorienting. slight cold, slight temperature, but nothing else. why?

Possibly infectious: It's possible that your cold symptoms represent a viral illness which is either directly or indirectly affecting your ear function either through the balance mechanism which we'd call a vestibular neuritis (caused by the virus) or perhaps your Eustachian tubes are a little inflamed or blocked which can also lead to feelings of dizziness or vertigo.