why is it i have to run my trans fluid level 3 litres overfull? on 2000 Volvo S80

the tcc (torque converter clutch) shifts in and out...(1700 rpm to 2000 rpm...back and forth) if i let the fluid level below 3 liters overfull..but high level seems to shift just fine....even 2-1/2 litres overfull it still kicks in and out. it HAS to be 3 litres over to not kick in and out....any suggestions???

Asked by for the 2000 Volvo S80