Does it sound like its pinking.Its probably blowing the spark out (det).Stick some new plugs in and make sure as they are gapped for your spec.Its the biggest killer of a Cosworth imo.My black one only did it once.Never keep your foot in if it does it because it will be game over,lean and a holed piston.Just to add I have been in cosworths and this happened with people who should of known better and the results were a meltdown.Any hesitation is a bad sign.