When I was writing service at a Ford store (HOLY %^#*, that was 20 years ago...I'm getting old), we had an Expedition that almost rolled into traffic on a major highway with the driver and her 3 kids. She said the brakes worked great slowing form 50 MPH down to about 5 mph, then the pedal went to the floor and it slowly rolled ahead (luckily she threw it into reverse and then Park). Turns out the ABS was getting a speed reading and thought the brakes were locking up, so it bled the pressure off until they started turning again. Not good when you're at a stop sign... Not clear from your post whether this was high or low speed, but just another possibility. I second checking for codes, but see if you can read live data to check sensors as well.