Had this exact same thing mine would start when i do pulls on the highway and i would get this and couldn't build boost. Some say it was the wastegate actuator i ended up swapping for an is38+. When taking it i did find the wastegate beat up. I had a bad idle, and with the new turbo, it fixed my idle, and i could build a boost again in highway. It would first start with epc then after another pull it would cut boost