Thanks for the reply. Indeed, I wondered about memory usage. It seems to hover around 40MB or so, which I think is fine as the collectors have 256MB, and don't do anything much other than run cflowd. The upshot is that when artsnetagg runs, the machine tends to thrash and things start to run pretty slowly. I wondered if this would cause a problem with cflowd.
Funny thing is, the aggregation scripts run in the early hours of the morning.
I have just checked the load on one of the busiest collectors. It seems to hover around 0.5 for most of the time except for some regular peaks where it shoots up to almost 2.0, before slowly falling off again towards 0.5. The peaks coincide with the writing of the stats to disk every 5 minutes. Maybe this much load is enough to cause problems.