Internal resistance goes up as they age. I think that is a big part of what eventually causes problems, even if the voltage still measures OK. One thing to remember if using a voltmeter to check for good battery voltage- measure while the battery is under load in the circuit, and not the open circuit voltage with a meter across the terminals and the battery unloaded. While under load the voltage of a battery at the end of it's useable life will drop significantly, but still measure okay out of the circuit.
A good charger with various modes that can measure the battery's actual capacity (by doing a few discharge and recharge cycles and reporting on the actual discharged power) and/or recondition older batteries and report their true remaining capacity helps manage them. I bought a better Maha charger after I had a few well-used but reliable NiMH die suddenly and/or refuse to run certain gear for more than a few minutes while running other gear for normal times (which I think had something to do with the internal resistance going up). That charger let me condition new batteries initially for best performance, match them in pairs by their true tested capacities, and monitor/recondition/discard them whenever I grow suspect of them or have a battery problem.