also try re pasting the boards and clean out the heat sink give it a good cleaning if you haven't, i had that happen once i took it a part and cleaned it up real good and reapplied the themo paste , that S5 had a 2 or 3 degree difference after i cleaned it up etc. .
and here is a S5 i kept and fixed this is not the same one i talked about above.. This one had a 4 to 5 degree difference after i fixed it . so i took it part once more did a really good cleaning re pasted etc..
now it runs better then it did when i bought it with one blade and replaced a blade etc .I'M also using S1 heat sinks they seem to work better then S5 heat sinks. and it also seems to run better over clocked then at defaults.