At 0.5amps for 60min, you're only putting in 500mah and not all of what is input winds up in capacity due to losses. That's not getting close to capacity when it peaks.

Charging at lower rates there is not as much voltage fluctuation which may be seen as a peak by your charger. Try charging at 0.5amps and if you don't get a highcapacity (mah) input, re-peak it a few times and see what the amount going in on repeak does. Does your 610c have a trickle option following a peak charge? You could use that to be sure they're topped off. When mine start getting old, I use a wall plugin charger and let them go overnite and then re-peak them on the charger before using.

Either that or start letting them charge at some low rate like 0.1amp for 14-16hrs. If you can't tolerate the time spent charging, its prob time for new ones.