Just thinking aloud... (and wild guessing)
Sticky HP valve?
Even though the engine may be producing enough (HP) pressure (upstream of the HP Valve), maybe the HP valve is sticking (closed) when cold, and IP is the only source of pressure(?) Is there any noticeable stickiness if you manually rotate the HP butterfly when cold? (Can you compare it with the other engine?).
On the other hand, it may be an HP Controller problem. Have you thought of purging the servo and sense lines. If the problem is related to operating temperature, maybe the HP Controller heater blanket is faulty (Do they have these on 757's?)
Regards
NSEU