Events notifications don't follow time delays

I’m a fairly new user but have run into an issue with events notifications. I set the delay to 5 or 15 minutes but after I click on the pop up notice, it pops up again in about a minute or two, sometimes a little longer. And it’s continuted to pop up after the event starts, although not every time. I checked settings and have “automatically dismiss reminders for past events” checked. It seems to me like the internal clock on these events doesn’t keep very accurate time so they pop up whenever. Maybe a bug that needs to be looked at.