here you go guys, this is the issue I had!
so yup, as others have said, i can only assuming this is a car bottoming out issue, as it's happened to me on parts of the track with bad bumps.
in the race it happened once, at eau rouge, when i took a wider entry into the right hand bit up the hill and there are harsher bumps - car 'jumped' and i realised not to venture out there again.
it does it at silverstone going into the maggots/beckets bit where there are some odd bumps there, car suddenly spazes out and you're flying onto the run off at 200mph.
i assume its a setup issue but i have zero idea how to fix it - this was my first ever online AC race with a pitstop (i had zero idea what to expect when i went into the pits, fortunately the menu that came up was quite user friendly!), with this car, for the setup i was just using softs and 65L for both stints, default setup other than reducing front/rear wing by 3 clicks, i softened both ARB's by 2 clicks, brake bias on 57, and that was it. i don't do setups, no clue!
so if you'd have left it on default you'd have been fine.