You do NOT want to put a big capacitor across the motor. Doing so will clamp the motor voltage and cause the H-bridge to dissipate much more power because there will significant voltage (Vsupply - Vcap) across the transistors.
When driving a motor with PWM, you are taking advantage of the inductance of the motor which permits instantaneous changes in voltage but forces there to be only gradual changes in current (dI/dt = V/L). If you put a C across the motor, you cannot instantaneously change the voltage without providing infinite current to the capacitor. Charging the capacitor forces the transistors to dissipate power. And that's ignoring L-C resonance effects you might get.
If you want it to work better:
- get rid of the capacitor across the motor
- don't use slow darlington devices as PWM switches
- definitely don't use darlingtons as emitter-follower
- use FETs instead (or an integrated motor-driver chip that includes FETs)