As mentioned your jaws may be splaying at that pressure so you are not getting good contact. Did you bore the jaws with a jaw ring at the same pressure you are using now.
If the jaws are a good fit you may be able to get similar metal removal rates by taking the rpm down to something like 2000 and increasing the feed to 0.25 ipr. The lower rpm will reduce the centrifugal force that is reducing the grip.
Or you may have to get a cenytrifugally compensated chuck. ($$$$$$$).
An open mind is a virtue...so long as all the common sense has not leaked out.