I ran more tests and the situation is the same for jobs with unknown sample rate as it is when the sample rate is identified. I can download jobs upto about 3.5 mins successfully then the error occurs for all jobs above that length.
The problem appears to be time related as a 10 min job fails after 30 secs at 50% progress while the 5 min job fails at around 75% after approx 30 secs? The download progress is never exactly the same when it fails which also points to a time out related cause?
Hope this helps.