Biting point changing on my disco 2 TD5

Hello was wondering if anyone could point me in the right direction. My clutch is playing up on my disco. It’s keep going from a some what decent biting point like just above middle of the pedal to right down to the floor when I go over 70mph for a duration of time. I’ve a new slave and master cylinder on within the last 2 weeks thinking that this would cure it. However it hasn’t. I’ve also bled it but to no prevail. Can not find any leaks from the pipe work and the level of fluid isn’t changing. Any advice would be very greatful