An intermittent problem is always much harder to correct than one which lights the warning light and it remains on. The shop you took the car to did the correct thing in replacing fluid and brake lines. That makes sense. Thing is with an intermittent problem my first step after changing fluid would be to scan the car for diagnostic codes. Most auto parts stores will do this for free. Hopefully that will produce a "stored" code that can get you headed in the right direction. Be aware that the Crossfire has by it's very nature a soft peddle for the brakes. It has been discussed many times here on the forum with no concise conclusion.