Starts at 87mph and continues until the end of the run, so the problem isn't married to RPMs, since you'd shift a time or 2 before the end of the track and would cover a wide range of RPM. It also takes a few seconds or minutes to clear up but clears up on its own. If you shut down at the end of a pass and read the plugs - are they still good? If you shut down at the end of a pass, will the car start and drive normally or do you still have to wait?
Does anything else happen around that time? Are you electronically controlling anything by speed or gear? Boost for example.
This all started when the belt shredded, right? Ever find root cause for the belt failure?
I'd look for damaged wires around the belt area. At lower speeds the frayed wire may be making enough contact to work, but once there's 100mph worth of air blowing on it, she opens up and loses connection. Slim chance, I know, but there isn't a lot going on that's speed dependent. No ABS or Traction Control stuff happening right? Those can be speed dependent.