What type of fuel pressure sensor are you using? If it is the VDO style, I would suspect it. The fact the pressure increases with the boost pump points to a sender reading errror (the VDO style senders usually fail reading high) or possibly a bad connection in the wiring or ground. If your mechanical pump were really pumping out 37 PSI (an atypical failure mode), it wouldn't likely jump higher from the 25 PSI coming from the boost pump.
Larry