Clean water has come up from my basement drain 3 or 4 times within the past 2 weeks. What could be the cause?

What happens when you sump pump is not working is that collection pit will fill to its maximum and water will spill out from the first upstream outlet. That may be your case.

You should make it a habit to remove the pump, hose it down, vacuum the pit and reinstall the pump. Run clean water to the pit using garden house and observe several cycles making sure pump in fully operational. Only then reinstall the cover.

Hope that helps... Millo