Wouldn't surprise me.....I wonder if the stream muxing is quite 'lumpy' ?
We have 3 muxes. The 2 that don't suffer this problem are [from a layman's perspective] administered and operated by the channel providers that broadcast on those muxes. For example, 1 mux is run by TVNZ, and that mux contains TVNZ's 4 channels. These other 2 are run at capacity, but not overly crowded.
This 3rd problem mux is different. It is a collection of all the channels from providers that only broadcast one channel each, and it is administered by a 3rd party transmission/communication company (Kordia).
I have no direct insight into the trunking, contracts, encoding, muxing etc. for the network ... but it has been my observation that the problem has emerged and worsened as Kordia have tried to squash more channels onto the mux. I think at last count there were something like 11 SD h.264 TV channels and 3 radio channels running within ~25 Mbps of bandwidth. They really do have to "optimise" the encoding parameters and statistical muxing to make it all work...