That is typically the problem with WiFi, if someone turns on a microwave or even a washing machine it can cut out your service from several seconds to several minutes at a time. It's even been known that some TVs can interfere with WiFi. How far you are from the router plays a big part as the further away you are the more interference can get in between you and the router.
If you require a reliable connection I would suggest using a cable or you may want to try using a powerline adaptor.