Yes, strange if PX1 fan slider locked,not on auto or custom profile.--I remember a similar issue from air cooled cards with earlier version of PX1--perhaps if you don't have at least PX1 V 1.1.7 ( 1.1.8 has RBAR VBIOS flash and maybe hold off on that one til you diagnose what's happening... ) maybe that solves it(?).  If it's not PX1 update, from your screenshot looks like RPM falloff from each peak at 71% back to ~63 is slightly different but in same narrow range suggesting perhaps somehow might be related to thermal throttling ( Might check rad & try monitoring temp and/or locking fan spd at higher setting temporarily to see? ) caused by another sensor reading or something interfering with one or bad sensor? 

I'm still not sure what's going on exactly when you have fan curve control enabled yet, but if auto setting works ok then seems as if hardware must be ok and issue must be caused by settings or profile issue somehow.  Try this: What happens if you drag all the points on the 3 custom profile graphs left to 0 temp with the top most point at 60% fan speed so line runs straight across from 0 to 100c where you wanted it to be in your 1st post, and change Temperature Hysteresis to 0 and save it as a new custom profile?  Does it now keep fan speed constant +/- a few rpm at 60% using fan curve control enabled?