Check EGR first.  It could be failing to work at that RPM, so make sure to test it completely.

Do you ever hear the engine pinging at higher RPM and load?  If so, you could have carbon deposits, a timing control issue, or something else causing detonation.

Are you saying you have a higher temp thermostat installed?  NOx is created at high temperatures, and cooling system "problems" can contribute to failure.

HTH