If flushing the DNS doesn't help, then either your wifi doesn't carry a strong enough signal or your router and / or wifi receiver sucks which is very common as most ppl buy cheap routers and adapters thinking they're all the same. I don't know your home layout and how far you are away from it and whether or not there's brick walls or a lot of walls in the way of the router but these days, the absolute bare minimum for any home should be around AC1900, and obviously the higher the number, usually the better the hardware used (doesn't necessarily mean you get better performance, but it does mean the internals are of better quality and therefore usually better reliability).
And as SylenThunder said... wifi sucks balls. Unless you have redundant wifi connections, reliability is always an issue, at least to some degree.