Engine should run nose down until it runs out of fuel. Was this 10 second test at WOT? If so, that may be normal. If at idle, that seems a bit short, but will depend upon the lenght of the lines. Check to see if the fuel line goes dry, and that is why the engine is shutting off. If the lines are still wet, then you have a mixture or some other issue.

If it is a dry line issue, the only good solutions are 1) don't do 10 second dives, and/or 2) use a header tank setup to extend the time you can be in a dive. and/or 3) use a bladder tank which will totally eliminate the issue.

Cheers

Edit: Probably not a clunk issue. If the clunk is forward, it would either die all the time (pinched line) or only nose up. It is shutting down when nose down.