Sudden onset of pvcs (4-6/minute) for 40 minutes, then totally gone. wasn't exercising. no stimulants or alcohol. could this happen in a healthy heart?

Yes : This can happen in structurally normal hearts. Usually, the PVCs are from the outflow tracts (right or left outflow tract). It's not uncommon for the PVCs to come and go with variable frequency and duration.

What?: How could you know that you were having PVc's? Do you mean palpitations? If so, then there it would not be clear that there were PVC's, since other rhythm changes -- and, no rhythm changes -- can be involved in palpitations. And it is quite possibly a health heart. No one can tell PVC's just by feeling: it takes an EKG.

Rhythm: yes, but make sure you are checked by a cardiologist, or electrophysiologist before writing that off.