My 2013 Sigma-engined 4/4 suffered similar intermittent problems to those you describe "begins to severely stutter or surge(clutch in the thing will rev out, or drop to idle(or stall" in the first few thousand kms of its life. Ambient temperature was not an issue as it happened in summer and winter.

My 4/4 would then run perfectly for hundreds of kms. But after it happened a hundred kms outside Sydney it caused me to lose my sense of humour and the dealer worked on it (still under warranty) until the problem was identified.

Eventually swarf, ferrous and non-ferrous, was found in the fuel tank. The fuel pump had been damaged by the swarf. A clean tank and a new fuel pump solved the problem.

Maybe this isn't your problem, but you may wish to consider it.

Good luck!

Oct 13 build 4/4