In general, those Amp Traps are extremely sensitive and should all be replaced when any of them go. It could be that you have been chasing around weakened fuses. If it was only at high speed I would suggest making sure the fields are weakening enough to keep armature current at nameplate. I had similar problems on a System 12 drive that ended up being a weak original equipment breaker that was left in the system before the disconnect. I would go through all the connections from the disconnect through the isolation transformer. A good thermal imaging camera is an excellent tool for this kind of problem. Maybe a bad contact in one of the contactors. It would show up quite a bit hotter than the others.