[Tom Line]

If you continue to have issues, consider doing this:

encoder_error = left_encoder_count - right_encoder_count;
encoder_output = encoder_error * encoder_gain;
if (encoder_output > 127)
encoder_output = 127;
else if (encoder_output < -127)
encoder_output = -127;

Left_Drive = 127 + <insert_your_left_drive_value> - encoder_output;
Right_Drive = 127 + <insert_your_right_drive_value> + encoder_output;

Adjust your gain until it drives roughly straight. It won't be perfect, but it might work well enough for you.