Do you have a separate feed pump or are you relying on the siphon created by the recirc pump? I could never get a steady flowrate using just the recirc pump. I guess this makes sense because the resistance will probably change as the media gets used up.

Anyhow my guess is if the media is a year old then this is very likely the culprit. A year is not a bad run. Seems to me an air bubble on its own shouldn't cause the pump to vapor lock (if you think about it, the pump should just suck that air bubble, shred it into a million little bubbles that trickle up through the media). I think it's more that the water has nowhere to go.