The car running at 1 rpm increases rapidly to 4 rpmWhen slowing or brake on

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Car has only 82,000 miles. Maintained. No problems. New tires once, new brakes, pads, etc once. About a year ago while braking to slow down car but instead car speed increased lunging forward. Increased brake pressure & it stopped. Never happened again until about 3 months ago & it's becoming frequent. I pulled into a parking place this week braked to park, with brake on, saw rpm' of 1 increase to 4 in a second jerking car forward with brake completely on. Also, drivers seat airbag light signal flashes on & off intermittently, but sometimes remains on.