It seems to be a common issue. If internet disconnects for a couple minutes or if stream goes offline for a couple of minites, thenn rtmpdump simply stops recording. Is there any free or cheap software that attempts to reconnect forever until manually stopped or something?
http://www.binarymark.com/Products/StreamingVideoDownloader/default.aspx may help. It works by monitoring traffic for rtmp content and when it detects the beginning of stream, it automatically records. As long as after Internet reconnection media server reinitiates streaming, it will be recorded (although a new video will be created for every streaming reinitialization).
Disclosure: I helped develop this product.