Due to dust etc. it could be that the fan is working less efficient, regularly cleaning your system on the inside would help on most cases. Replacing the Cooling paste in between the GPU and the Cooler itself can also make a big difference.
When removing the cooler from the board please warm up the card first by using it for about 10 minutes. This makes it easier to remove the cooler. Romove old cooling paste with a tissue ro something like that , and aplply new paste only on the the GPU (dont let it spill off the GPU so you wont need that much paste).
Asol AMD drivers after 16.8.3 have many adjustments for RX400 series but to my knowledge nothing much for HD7800 series, so it is very well possible that your card will run better on a “older” driver like the 16.8.3.