Try a fresh driver from Lenovo "Broadcom 802.11ac Network Adapter 7.12.39.11": http://support.lenovo.com/us/en/products/laptops-and-netbooks/lenovo-z-series-laptops/lenovo-z50-75//downloads/DS100424 unzip and select: WLAN Driver (Atheros, Realtek, Broadcom, Intel)\Broadcom\7.12.39.
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