If you've gotten to the point that the battery is shutting down, then you can start the charging calibration. Sounds like you have a crapping battery, but try a calibration charge.
I'm not sure of the correct term, but the battery has a "hysteresis" when draining under a load. Once the load is removed and/or that the battery cools, the voltage will rise a certain percentage...maybe a single volt, just depends.
As to how to drain, several threads here prescribe various methods, just depends on what is easiest for you. I use a series of 12vdc auto lamps, about 160W worth, to drain the batteries down to 50% for storage. I guess the same tool could be used for re-calibration discharging.
When discharging, the individual cells should be monitored rather than an overall voltage. 12v divided by 4 cells is 3vdc per a cell. If you have a "deader" cell you could easily damage it further by taking it below 3vdc. 3.1 + 3.1 + 3.1 +
2.7 = 12
I think
Solex has the best method, at least from what I've read. If you don't have Solex, then you have other options...search is your friend.