I was having the same probs with my IB3800`s & IB3600`s (old), charging with a Supernova Competition @ 6amps & 4mv, discharging @ about 20amps with bulbs & then traying with a Smart Tray to 0.9v.
An electrician racer mate suggested to up the delta peak to around 10-15mv being old cells.... & they are now taking around 4000 to 3900 respectlively.
So maybe give that a try
FT TC4, LRP TC Sphere, LRP X-11 13.5
M03M Suzuki Swift
Power by Feral
Sanwa MX-3 FHSS