Possible bug in HCE

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On my Hardware I get about 500 KNPS on average with DeepHiarcs 14 analyzing most middle game positions ....(I am using both cores of my computer) ...I have noticed that however a lot of times the NPS drops to around 300 KNPS (which is a huge drop , staying on the same position) , and stayes there for a few seconds , before going back to the Usual 500 KNPS (in a split second)...This is very unusual for a chess program and happens a lot too , so I am wondering what is going on here......Operating system is windows XP.........