hey mate

sounds like one of those annoying ones. If ur saying it only seems to happen when the car is being driven hard, have you run it up to temperature, and then tried giving it some revs from under the bonnet?

there is other ways aswell. Pressure test the cooling system?

Or if you cant do that, you could try this as a lest resort although i dont like suggesting it, disconnect the fan switch plug, then run it up to temperature, and it will get that little bit hotter when the fan shgould of cut in, and try to boil at which point the pressure in the system will be higher and you are more likey to find the leak. Be careful though if you leave it too long it will boil over. It shouldnt cause any harm as such as you wont be leaving it to boil over for a long time, just be careful though?

let me know if you want anymore help mate