NTLM with GTX 980

[Serpent]

I just bought my new GTX 980 card, on bench marking it gave a speed of aprox. 21500 MH/S , when i tried using oclhashcat with a real NTLM hash the speed keeps changing between 18000 MH/s and 14000 MH/s .. so does the util. which drops to between 25% and 80% .. i trieng to figure whats wrong .. could it be im getting the rule wrong ?