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TMS320F28069M:Unstable motor control

I use the dual motor routine of lab12c, incremental encoder, the motor parameters are:

#define USER_MOTOR_TYPE MOTOR_Type_Pm
#define USER_MOTOR_NUM_POLE_PAIRS (24)
#define USER_MOTOR_Rr (NULL)
#define USER_MOTOR_Rs (0.189)
#define USER_MOTOR_Ls_d (0.00038)
#define USER_MOTOR_Ls_q (0.00038)
#define USER_MOTOR_RATED_FLUX (0.25)
#define USER_MOTOR_MAGNETIZING_CURRENT (NULL)
#define USER_MOTOR_RES_EST_CURRENT (2.0)
#define USER_MOTOR_IND_EST_CURRENT (-1.0)
#define USER_MOTOR_MAX_CURRENT (40.0)
#define USER_MOTOR_FLUX_EST_FREQ_Hz (20.0)
#define USER_MOTOR_ENCODER_LINES (4096.0)
#define USER_MOTOR_MAX_SPEED_KRPM (0.28)
#define USER_SYSTEM_INERTIA (3.85)
#define USER_SYSTEM_FRICTION (6.96)
#define USER_MOTOR_INERTIA (0.652)

In the process of use, sometimes the dual motors are very stable, and sometimes there will be unstable phenomena (the motor movement has a wavy feeling, and pulling the motor will feel a period of strength and a period of weakness). To solve this problem, is this a problem of the motor itself, or is it a code control problem? If it is a code control problem, how can I solve it? In the dual motor routine of lab12c, are there any variables that can be observed and monitored and explain the cause of this problem?