I don't think there is. To recap, "sometimes the bandwidth on the encoder side is congested for 1 or 2 seconds, so the stream is not so stable from the encoder to the icecast server". In a Flash publishing scenario, you would increase the NetStream buffer in the encoder app. I don't think Wowza has any way of fixing this.
Compare different settings, find out if you can get any benefit from changes in these settings, there are inline comments indicating usage, and there is a configuration reference in the documentation folder. If data is just missing as a result of this upstream congestion, as I suspect, they won't help.