with SJ units, they're generally pretty ok but occasionally gotta watchout for:-

1, dry joints on PCB... some go bad after bumping your charger around field/track
for a few years. Could be worth opening up case and reflowing all the joints.

2, you may have a dodgy balance tap board, if you're still using the stock original.
Could be a bad pin or wire, or solder joint has gone dry - take a close look.
(If a middle wire loses contact then its neighbour will suddenly read double voltage.)

You can buy spares but I also find it very short & stressful for the wires when
you need to pull it all the time, so i like to replace the plug-in board totally with a
custom-made cable. The plugs are common Molex connectors, i like to wire 1
for each type of battery and be long enough to reach where i want the battery to be.