I've seen roughly the same clocks as you but as I have Samsung memory I've been able to add 750 onto it and it stays there quite happily, Only seen the slowing down thing of the fan once for a split second but I have a feeling it has something to do with GPU load as I only saw it once when 3DMark was switching scenes, Very happy with this card though
What are your ambient temps ? I have a 2080 Ti blower from Zotac, Objectively worse cooler than the FE cooler but the same PCB, Die, Component layout etc... just a different cooler and it hovers around 72'c at 1900MHz with a 1:1 fan curve, To me it sounds like you've got bad contact between the die and cooler, Just RMA it.