My first 7900XT (XFX merc black) was almost exactly like that. Amazing performance out of the box getting up to 2900Mhz-3100mhz but about a month after using it and some driver updates the clocks went down to ~2700mhz with the hotspot delta being 35-50c (it was hitting the 110c limit). I had to contact XFX and they said it was alright for me to repaste the card and I replaced the torn pads as well and it helped for another 1-2 months but it ended up with the same issue so I had to RMA it and my new card has lower clocks on average but the delta between hotspot and average temp is only 5-10c under med load and 15-20 under full 400watt load. I have tested it without my side panel being on and it drops it a few degrees so for me it is just my case being not big enough to handle the full wattage that it can pull. (which is why I now have my card at -10% power limit and of course undervolted).
So you could have a few issues here, 1 the cards cooler might be bad like my first card or 2, you might not have enough space in your case for the amount of heat it will pump into it. A simpler solution to make sure it isn't the drivers messing with your card is to use DDU and go into safe mode to uninstall the AMD drivers and then install the last known "good" drivers. If your card goes back to normal than it is something software related, if it doesn't then see if it is the case or the heatsink itself.
Hope this helps.
