Ajax failing multiple times before it works - php

Question

I have a very annoying bug. My AJAX queries are failing anywhere up to 20-30 times with 500 GET and POST server errors, but then all of a sudden they work.
It seems to be completely random, as sometimes I will load the page and it will work fine for a full day of usage, but then the next day it will fail and I have to catch and retry it up to 30 times to get it to work.
AJAX
function getData(){
//Get Data and build contributions Table
$.ajax({
url: 'assets/processes/dash/support/get-data.php', // URL of php command
tryCount : 0,
retryLimit : 3,
type: 'POST', //TYPE
data: {'id': id}, //Variables in JSON FORMAT
success: function(results) { //SUCCESSFUL REQUEST FUNCTION
var result = $.parseJSON(results);
console.log(result);
},
error : function(xhr, textStatus, errorThrown ) {
if (textStatus == 'timeout') {
this.tryCount++;
if (this.tryCount <= this.retryLimit) {
$.ajax(this);
return;
}
return;
}
if (xhr.status == 500) {
$.ajax(this);
return;
} else {
//handle error
}
}
}); // end ajax call
PHP
<?php
include "../../../includes/database.php";
$totalOpen = $db->pdo_query_assoc("xdGetTotalUnresolvedTickets
1,0,0,0,0,0");
$totalWaitState = $db->pdo_query_assoc("xdGetWaitState 1,0,0,0,0,0");
$nonWaitState = $totalOpen['xdGetTotalUnresolvedTickets'] -
$totalWaitState['xsGetWaitState'];
$getStaff = $db->query("SELECT * FROM user WHERE role = 'Technical
Support'");
$getStaffArray = array();
foreach($getStaff as $key => $value){
$sqlUsername = $value['sqlName'];
$pdo_today = $db->pdo_query_assoc("xsGetTotalResolvedTicketsToday
'$sqlUsername'");
$start = date('Y-m-d ', strtotime('-7 days'));
$end = date('Y-m-d ', strtotime('+1 days'));
$pdo_last_week = $db->pdo_query_assoc("xsGetTotalResolvedTicketsToday
'$sqlUsername','$start','$end'");
$getCompletionRate = $db->pdo_query_assoc('xStaffTicketCompletionRate
"'.$sqlUsername.'"');
$getWeeklyRate = $db->pdo_query_assoc('xLastSevenPreviousSeven
"'.$sqlUsername.'"');
$getStaffArray[$key]['staffName'] = $value['user'];
$getStaffArray[$key]['staffFullName'] = $value['name'];
$getStaffArray[$key]['colourScheme'] = $value['colourScheme'];
$getStaffArray[$key]['today'] =
$pdo_today['xsGetTotalResolvedTicketsToday'];
$getStaffArray[$key]['thisWeek'] =
$pdo_last_week['xsGetTotalResolvedTicketsToday'];
$getStaffArray[$key]['completionRate'] = $getCompletionRate['Level'];
$getStaffArray[$key]['thisTimeLastWeek'] =
round($getWeeklyRate['Percentage'],1);
$getStaffArray[$key]['weeklyRateLevel'] = $getWeeklyRate['Level'];
$getStaffArray[$key]['weeklyPrevious7'] = $getWeeklyRate['Previous 7
Days'];
$getStaffArray[$key]['weeklyEarlier7'] = $getWeeklyRate['Earlier 7
Days'];
}
$tickets = array('totalCurrent' => $totalOpen, 'totalWaitState' =>
$totalWaitState, 'totalNoneWaitState' => $nonWaitState);
$allData = array('totalTickets' => $tickets, 'staff' => $getStaffArray);
echo json_encode($allData, JSON_FORCE_OBJECT);
?>
I know at the moment the trycount doesnt work properly, as instead of trying 3 times it tries unlimited amount until it works. Any one have any ideas as to why this could be happening?
They always fail on the php file, like the query cant be executed.
ERROR
GET http://mysite/get-events.php 500 (Internal Server Error)
jquery.min.js:4