Harry,
I remember back in the days of racing the XU1 in South Island Production Saloons there was a chap who had a wickedly fast E49 Charger that used to suffer from cracking discs.
His solution was to either cover the wheels/ tyres with sacks or later simply had plywood inserts that would sit inside the mags and stop the air moving through them so they would cool down very slowly.
Hope this may be of some help.
Cheers
Dave