[86Meesta2]

Could use some outside opinions- am having a strange clutch problem with my '99 996:

Two days ago I thought I felt some strange behavior in the clutch. When driving again today I confirmed that something doesn't appear to be right. ONLY when driving, the clutch pedal will get soft to ~1/2 travel for one stroke if I don't use the clutch over the course of a few minutes. It ultimately still engages, but offers almost no resistance until ~halfway down. When I cycle a second time right away, the pedal is instantly solid again. In stop and go driving nothing appears wrong, but when cruising on the highway or side roads without shifting (no influence from gear selection) that is when it manifests itself. The strange part is the pedal remains rock solid when sitting idling or when the car is off (engine hot or cold).

Time history of recent work:
I replaced a failing slave cylinder ~1 mo and 1000 miles ago. Also replaced the master at the same time. New DOT4 fluid. Seamless performance from install until this


Troubleshooting done so far:
-With the car off, I have held the pedal part way for several minutes but the pedal remains solid. Whenever the car is off, including sitting overnight, the pedal is solid on startup. Thought was if there was a leak holding pressure in the system would cause it to bleed down
-Bled all fluid, had a thought about moisture in fluid leading to lowered boiling point (unlikely given dry climate but easy to replace)

I'm usually good at troubleshooting but am at a bit of a loss on this. Feasibility of some type of internal master/ slave failure? Pressure plate fingers cracked? Disc worn? Clutch assist spring gone bad? (but why the soft pedal only when driving)