"Slowly but surely..."  I'll need BSC8 access in the morning and again tomorrow evening.  

Check out the attached image.  This looks like a step response that couldn't be the result of a real pressure change.  I would guess it has something to do with the shared 24 DC supply between PT180 and PT120B and that pulling the fuse for PT180 changed the supply's load - or something - but nothing related to heating the PT180 hardware.