Strange HTTP corruption errors at high load with Gatling


I’m seeing very strange behaviour with Gatling on Windows / Java 8 under high concurrent load (60 requests / sec).

I’m getting HTML content where HTTP status lines should be.
The same scenario runs fine at lower load (10 requests / sec).

I have put full details up on Stackoverflow here:

Has anyone seen anything like this before?
Does anyone have any hints on how to diagnose and fix?