Sporadic data from CAN-bus Seed shield using aftermarket ECU

I have a strange problem with a project im working on. Im connecting a Seed Studio CAN shield to an aftermarket car ECU. I modified some source code i found on GIT for the specific ECU, but for some reason i dont always get any data. If i reset the Arduino a few times in a row it usually starts printing to the serial monitor, i can confirm that once i receive data it’s always correct.

When i switched on the igntion (this is a race car so just a switch to turn the ECU on and a button to crank engine) i never get any data unless i keep resetting the Arduino. The arduino is powered up when i turn the ignition up, so ECU and Arduino starts at the same time, but i use voltage converter to make sure the Arduino only get 5v.

I tried to power the Ardunio directly from USB from the computer and then power up the ECU then the data usually appears in the serial monitor directly at least 9 times out of 10

Any ideas why this is happening? I did a serial print in the each case and it never prints out anything when i dont recieve any data so for somtimes it does not get the correct identifier.

The ECU i sends with 500 kbit and the protocol used in this example looks likes this for in this example RPM, engine temp and manifold temperature (AIM variant 2 protocol)

ID A ID = 0x770
Byte 0:1 =RPM

ID B = 0x771
Byte 0:1 = CLT

ID C = 0x772
Byte 2:3 = CLT

Is there something i have missed? Im suspecting im getting corrupted data sometimes

#include "mcp_can.h"
#include <SPI.h>
#include <stdio.h>

#define INT8U unsigned char

const int SPI_CS_PIN = 9;

INT8U can_len = 0;
INT8U can_buf[8];

INT8U ecu_error = 0;
INT8U ecu_throttle = 0;
INT8U ecu_rpm = 0;
INT8U ecu_clt = 0;
INT8U ecu_mat = 0;
int ecu_map = 0;

float ecu_lambda = 0;
float ecu_volt = 0;

void setup() {
  attachInterrupt(0, MCP2515_ISR, FALLING);

void loop() {

// Interrupt Display Data
void updateLCD() {

  // Copy data to Serial1
  Serial.print("U:   ");
  Serial.print("CLT: ");
  Serial.println(ecu_clt, DEC);
  Serial.print("MAT: ");
  Serial.println(ecu_mat, DEC);

// Interrupt CAN Data Reseived
void MCP2515_ISR() {
  CAN.readMsgBuf(&can_len, can_buf);  

// Parse data from buffer
void parseData(int id) {
  switch(id) {

  case 0x770:
    ecu_rpm =  ((int)(word(can_buf[0],can_buf[1])));

  case 0x771:
    ecu_clt = ((int)(word(can_buf[0],can_buf[1]))/10);
    ecu_volt = ((float)(word(can_buf[4],can_buf[5]))*5/1024);
    ecu_throttle = ((float)(word(can_buf[6],can_buf[7]))/10);

  case 0x772:
    ecu_map = ((int)(word(can_buf[0],can_buf[1])));
    ecu_mat = ((int)(word(can_buf[2],can_buf[3]))/10);
    ecu_lambda = (int)(word(can_buf[6],can_buf[7]));

  case 0x773:
    ecu_error = (int)(word(can_buf[4],can_buf[5]));