The error correction on the ABC channels is quite low, just 9/10 I think from memory. That means if there are any glitches, any interference or the signal falls just a little the receiver has a very small margin before it loses signal.
The FEC number is very important when you're trying to pull in a channel as a larger number (1/2 or 2/3) has much more margin before signal is lost. I think NHK World on 58W is FEC 1/2 and as a result you can pull it in with a small dish.
On ABC, the feeds are meant for broadcasters with much bigger dishes so things get dicey on a smaller dish. It's a clever way to stop too many people from viewing the broadcast without encrypting and a more efficient use of bandwidth.
In your case, it seems your reception is right on the edge and every few seconds your receiver loses its lock.