Microsoft Graph API (Calendar) Intermittent 503 Errors

This code sometimes works, but frequently runs for ~20s then fails with the "503 Service Unavailable" message when I call getPage(). The authentication/token request seems to be working fine.

I can't seem to identify any pattern of when it fails/succeeds. I don't believe it's a throttling error, as there is no "Retry-After" header returned, and the script only runs once per day at night with <200 records returned. I've also tried removing the $filter and changing parameter order as described here, with no clear benefit.

Can someone please help find the cause here? Happy to share any additional info. Any help is much appreciated, thanks!

<?php
define('DEBUG', true);
session_start();

// set config vars
$ms_url_base = "https://login.microsoftonline.com/d3523db7-f84a-4a24-a815-cd4ba4691c9c";
$ms_client_id = '<client id>';
$ms_redirect_uri = "https://example.com/path";
$ms_scope = "calendars.readwrite user.read";
$ms_client_secret = '<secret>';
$ms_auth_endpoint = '/oauth2/v2.0/authorize';
$ms_token_endpoint = '/oauth2/v2.0/token';
$query_numdays = 100;
if (DEBUG) error_reporting(E_ALL);
date_default_timezone_set("America/Detroit");

require_once __DIR__.'/vendor/autoload.php';

use Microsoft\Graph\Graph;
use Microsoft\Graph\Model;
class EventMod extends \Microsoft\Graph\Model\Event {
    // custom functions here 

    public function getNextLink() {
        parent::getNextLink();
    }
}
class ResponseMod extends \Microsoft\Graph\Http\GraphResponse {}

// authorization
$provider = new Stevenmaguire\OAuth2\Client\Provider\Microsoft([
    'clientId'                  => $ms_client_id,
    'clientSecret'              => $ms_client_secret,
    'redirectUri'               => $ms_redirect_uri,
    'urlAuthorize'              => $ms_url_base.$ms_auth_endpoint,
    'urlAccessToken'            => $ms_url_base.$ms_token_endpoint,
    'urlResourceOwnerDetails'   => 'https://graph.microsoft.com/v2.0/me',
]);

if (!isset($_GET['code'])) {
    $options = ['scope' => $ms_scope, 'aud' => 'Graph'];
    $authUrl = $provider->getAuthorizationUrl($options);
    $_SESSION['oauth2state'] = $provider->getState();
    header('Location: '.$authUrl);
    exit;

} elseif (empty($_GET['state']) || ($_GET['state'] !== $_SESSION['oauth2state'])) {
    unset($_SESSION['oauth2state']);
    exit('Invalid state');
} else {
    try {
        $token = $provider->getAccessToken('authorization_code', ['code' => $_GET['code']]);
    } catch (League\OAuth2\Client\Provider\Exception\IdentityProviderException $e) {
        exit ("Get access token exception: ".$e->getMessage());
    }

    if (DEBUG) {
        echo 'Access Token: ' . $token->getToken() . "<p>";
        echo 'Refresh Token: ' . $token->getRefreshToken() . "<p>";
        echo 'Expired in: ' . $token->getExpires() . "<p>";
        echo 'Already expired? ' . ($token->hasExpired() ? 'expired' : 'not expired') . "<p>";
    }

    // start calendar query
    $start=new DateTimeImmutable("yesterday 0:0:1");
    $end = $start->add(new DateInterval("P".$query_numdays."D"));

    $url='/me/calendarview'
        .'?startdatetime='.$start->format('c')
        .'&enddatetime='.$end->format('c')
        .'&$filter=isOrganizer+eq+false'
        .'&$select=subject,responseStatus,start,categories'
        .'&$orderby=start/dateTime';

    $graph = new Graph;
    $graph->setAccessToken($token->getToken());
    $data = array();
    try {
        $iterator = $graph->createCollectionRequest("GET", $url)
                    ->setReturnType(EventMod::class)
                    ->addHeaders(["Prefer" => 'outlook.timezone="America/Detroit"'])
                    ->setPageSize(25);
        do {
            $page = $iterator->getPage(); /*************** THIS IS WHERE THE EXCEPTION HAPPENS ************/
            if (DEBUG) echo "<pre>".print_r($page, true)."</pre>";
            $data = array_merge($data, $page);
        } while (!$iterator->isEnd());
    } 
    catch (\Microsoft\Graph\Exception\GraphException $e) {
        if (DEBUG) echo "GraphException Message: ".$e->getMessage();
        exit;
    }
    catch (Exception $e) {
        if (DEBUG) echo "Unk Exception getting data: ".$e->getMessage();
        exit;
    }

    if (DEBUG) print_r($data);
}
?>

composer.json

{
    "require": {
        "microsoft/microsoft-graph": "^1.29",
        "stevenmaguire/oauth2-microsoft": "^2.2"
    }
}