Yeah, I was going to say freehub. If the problem comes back, though, try wiggling the wheel side to side with your hand with the bike sitting still, wheel on the ground. Is there a lot of lateral play? This is different than the wheel being out of true. It might be that the outsides of the hub itself has to be tightened (not the freehub). I'm told this is fairly common with Mavic wheels.
Some Easton hubs will also do this, but it's more difficult to do it, and it's not a long-lasting fix (eventually the whole hub needs to go).

Mavics do need to have the freehub pawls regreased pretty regularly. How old was that freehub? Just curious to see how much life you got out of it.