I have an 2001 S4 with a very intermittent brake problem. Maybe twice a year the left front brake will apply itself, sometimes with enough force to cause the tire to squeal and I could feel the pulsing of the ABS. The last time, I was traveling at a steady 45 MPH and not touching the brake pedal. The ABS and ESP warning lights will usually come on after this happens. They go out when I restart the engine When I take it to the repair shop, it shows no error codes. What can cause this strange behavior? Thanks.