|If the card has a fan then be sure the fan is free of dust and is rotating properly. You also need to clean the entire insides of your case and power supply. The hotter it is inside there, the hotter the card will run even if working right.|
The best method to clean out the insides,IMO, is to blow it out using canned or compressed air. DO NOT use a vacuum cleaner due to electro static discharge. Brushing is usless, IMO.
Install HWMonitor from the link below and run it in real time to determine the temperatures and voltages under load. Get HWMonitor at the link below.