Geoff, I'm basically with Bruce on this, but I would just go along measuring the voltage on each battery (I'm a lazy bugger), should reveal any failing battery.
That said, Dinx has a relevant point, but you tell us the bank is around 5 years old, so taken as read, tell us about your charging, is a regulator controlling this, for how long do the receive 29 volts?
You equalize regularly, how often is regular and at what voltage? Do the batteries have clear cases such that you can see inside? You are feeding them but are you watering them or are they sealed/gell?
My original bank is now 20 years old, was 13 years old when I received (stole) them, they were arranged in 8 banks @ 12 /bank but were only 650 A/h each, now half have fallen by the way side, the voltages you mention are typical of my batteries.
Allan