Sounds like your display needs a dusting out!
These are great displays! But over many years of use the dust buildup inside can interfere with its cooling. You may need to replace its fan if the bearings are gone Apple P/N 922-9358.
To open the display its just like the 2011 27” iMac, follow this guide [guide|7551]
Use a small soft paint brush to help scrub the surfaces to loosen the dust and then use can’ed air to blow it away.[br]