I think you are still using the 20 yr old harness after the build IIRC. One of the local Z guy has this intermittant bogging/fall on its face issue last time. One moment he could be boosting happily and the next the car just stumble and basically running on 3 cylinders. But everytime he got the car to me to look at, it was running 100% no issue at all. Then he took it back and few days later it happen again. No code were found.
Turned out a few coil pack connectors and injectors connectors were all corroded/broken/loose. He got a 90tt so it was 20yr old harness. We swapped out new connectors and I have not hear a word from him about bogging.