I have a Z370 and it does the same random keyboard problems you all mention.
I did discover that when it starts happening if I open up the On--Screen Keyboard and use it for a bit, the problem corrects itself.
i finally called support last night, he didn't try very hard to fix it. said they got calls every day about it. told me ot send it in. i dont want to be without it for a week-10 days. he said buy the on site warranty and in 30 days a technician will come to my house to fix it. my thoughts are i wasted my money on the on-site warranty. and wasted my money getting a lenovo.
keep hoping i'll come across some sort of solution, other than having to stop and open up the on-screen keyboard.