AWS HTTP API Gateway + Lambda ( Node/express) 503 Service unavailable

I don’t have much knowledge in AWS

I have following setup,

const express = require("express");
const serverless = require("serverless-http");
const app = express();

app.use(cors());
app.use((req, res, next) => {
    res.setHeader('Connection', 'keep-alive');
    res.setHeader('Keep-Alive', 'timeout=30');
    res.setHeader("Access-Control-Allow-Headers", "X-Requested-With,content-type");
    res.setHeader("Access-Control-Allow-Origin", "*");
    res.setHeader("Access-Control-Allow-Methods", "GET, POST, OPTIONS, PUT, PATCH, DELETE");
    res.setHeader("Access-Control-Allow-Credentials", true);
    next();
});

app.use(express.json());
app.use(express.urlencoded({ extended: true }));

app.use(`api-end-point/user`, userRoute);
....

if (process.env.NODE_ENV !== "lambda") {

    PORT = process.env.PORT || 7000;
    const server = app.listen(PORT, () => {
        console.log(`node-express server running in ${process.env.NODE_ENV} mode on ${PORT}`);
    });
    server.timeout = 0;

}else {

    module.exports.handler = serverless(app);

}

user.controller.js

const AWS = require("aws-sdk");
const CryptoJS = require("crypto-js");
const Base64 = require("crypto-js/enc-base64");
const config = require("../core/config/config.json");

...
...

async getUser(token, cb) {
        var params = {
            AccessToken: token /* required */
        };

        try {
            this.cognitoIdentity.getUser(params, (err, data) => {
                if (err) cb(err); // an error occurred
                else cb(null, data); // successful response
            });
        } catch (e) {
            console.log(error);
            return false;
        }
}

And of course, I have cognito user pool from where I get the requested user information using above getUser by passing Authorization token (after login).

Local Environment

When I run this and all other APIs, believe me I haven’t received any error single time and all APIs work just fine without any problem.

API Gateway + Lambda Enviornment

Problem happens when this code goes to Lambda function.

API gateway routes are configure as /ANY, /{proxy+}

Also, CORS setting is as below,

And the entire code goes to LAMBDA function and lambda is associated with API gateway.

When I hit `API gateway + Lambda (+ congnito)` to get **user information**. It works fine. Most of the time, it works but there are instances where it fails and returns (all of sudden)

503 Service Unavailable

Now, I really have no idea what is going wrong.

TIMEOUT

API gateway timeout is set to default which is 30 seconds.

Lambda timeout is set to 10 MINUTS. (I just increased lambda timeout to try something)

1. BUT THIS ISSUE KEEPS OCCURING

2. Sometime (NOT Every time) I get CORS issue (out of no where and all of sudden)

3. Is there any way I can increase API gateway timeout

What should I real do to make work every time ?

NOTE: getUser function is very straight forward. I don’t think it is taking so much time to get user details from cognito user pool. something is wrong which I’m not able to figure out.

Please please help