amd a6-6310 capping at 900mhz

ive been haveing an issue with an older pc i stll oftern use where the cpu is capping around 500-900 mhz and it normaly runs at 2400mhz its been causing really bad lag on games i think its becuse the fans arnt reporting back however they are running. i know that the cpu caps if it gets t ohot but its been running at 40 Celsius is there some way i can over ride whats causing the cap or manualy control what the cpu runs at so i can keep it running above 1.6ghz as it use to run at 2.4ghz fine and get to the 75 Celsius before getting upset.changed fans and cleaned insides out