FBO and they warrantied your turbos? Did you take off the DPs prior?
I had this same issue only when it was really hot out last summer. We had temps around 100 and it would happen at 4k only. When the temps dropped it went away, but it seems that it could be with an issue related to where the downpipes bolt up to the mid pipes. Downpipe flanges being off. BuraQ had this issue as did one other member (besides me).
2011 Mineral White M3 ZCP
Paint Correct, Opticoat, Ventureshield, some CF, B&B exhaust, AFE Intake