Engine/Transmission (1998.5 - 2002) fuel pressue rising

[CEpp]

fuel pressure rising and keeps rising when sitting still at idle, starts at about 11 psi and have seen it go as high as about 17 psi, shut truck off and restart and its seems ok and back at 11-12, or drive it and takes a little time but eventually it will come super slowly down to 11-12. think it could be a isolator prob, but could it be the overflow valve sticking?