Sorry, I misread your original message. I thought the problem was occuring for the first 15 minutes of engine ops rather than after 15 minutes
so I believe HPSOV is opening.
But it seems to be closing, erroneously, after 15 minutes. If the lines and the HP valve have been checked/changed, then I guess you have to go further up the chain (HP Controller, ASCTU or even an ADC). I'm assuming the 757 is similar to the 767, in that one of the ADCs provides the altitude signal (only using the altitude switch as a backup).
Unfortunately, if the FIM is not helping, it may be a matter of replacing the system piece by piece
or calling RR or Boeing to see if they have this type of problem on record.
Cheers
NSEU