OK, I’ve been thinking and here is what I think might be happening.
For whatever reason the pump is putting out little or no water. If the water level definitely isn’t below the level of the inlet tube, then there must be some other reason that’s reducing the water flow. This could be a kinked tube, a split in the tube, a blockage etc. but the strange thing is at this is happening intermittently. You need to take the case off, and have a good look around to see if you can find the cause of this behaviour and why it might be intermittent. Are there any other strange symptoms which you didn’t think worth mentioning?