need help..

i am using MOBO=MSI 870-G45 | PROC= athlon x 2 255(cooler master geminII) | GC= XFX 5770 | PSU =CORSAIR GS 600watt..

using button AUTO OC GENIE,i oc to 20% when i turn on my pc can run normaly..but when i run something heavy it will be bluescreen....
before this i using normal GC msi 210,oc to 20% n then turn on my pc it running normal n i can play anything i want without blueescreen..after i change my GC i cant OC to 20%..if i oc to 20%it will be bluescreen..right now i only oc 15%..realy need help to solve this problem ..why i can oc to 20%???

(sory for broken english)