It's happening because there is not enough space on gauges area of the page to display the pop-up chart and the cursor by its side. The cursor is place over the chart, which means it is no longer over the gauge, so the chart is hidden - rinse and repeat!
Two options:
1. Reduce the displayed size of the charts.
2. Change the position of the cursor relative to the chart so it no longer overlays it.
Option 2: By default the cursor is 30 pixels down by the side of teh charts, change so that it is always slightly above the chart...
gauges.js, line 4242
Change...
tooltipoffsets: [20, -30], // additional x and y offset from mouse cursor for tooltips
to...
tooltipoffsets: [20, 10], // additional x and y offset from mouse cursor for tooltips