Random Queries Failing with MySql - php

Question

I hope this question is not too broad but it is not with a specific piece of code. Basically at random times and on random queries I have failures in my code. Most of the time it seems like it is on my INSERT calls. UPDATE and DELETE still work fine, but INSERT will fail across the entire page for several hours at a time before mysteriously working again. The page is only being used and tested by myself currently.
One of the sample queries.
PHP
session_start();
$poster = $_SESSION['login_user'];
$conn = new PDO("mysql:host=localhost;dbname=spectrum",'root', '1234abcd');
$u = $_POST['user'];
$p = md5($_POST['pass']);
$e = $_POST['email'];
$fn = $_POST['first_name'];
$ln = $_POST['last_name'];
$t = $_POST['type'];
$sql = "INSERT INTO users(id, user, pass, email, type, first_name, last_name, poster) VALUES ('', :user, :pass, :email, :type, :first, :last, :poster)";
$q = $conn->prepare($sql);
$q->bindParam(":user", $u);
$q->bindParam(":email", $e);
$q->bindParam(":pass", $p);
$q->bindParam(":type", $t);
$q->bindParam(":first", $fn);
$q->bindParam(":last", $ln);
$q->bindParam(":poster", $poster);
$q->execute();
echo json_encode('User has been added.');
This is done through an Ajax call.
JQuery
var request;
if (request) {
request.abort();
}
var $form = f;
var $inputs = $form.find("input, select, button, textarea");
var serializedData = $form.serialize();
$inputs.prop("disabled", true);
request = $.ajax({
url: site + "/backend/formup.inc.php",
type: "post",
dataType: 'json',
data: serializedData
});
request.done(function (data){
if(data.location){
window.location.replace(data.location);
}
else{
alert(data);
location.reload(false);
}
});
request.fail(function (jqXHR, textStatus, errorThrown){
alert(
"The following error occured: "+
textStatus, errorThrown
);
});
request.always(function (data) {
if(!data.tvar){
$inputs.prop("disabled", false);
}
});
Here is the HTML.
<form class="hidden" method="POST">
<input type="text" name="user" placeholder="Username" required/>
<input type="email" name="email" placeholder="Email" required/>
<input type="password" name="pass" placeholder="Password" required/>
<input type="text" name="first_name" placeholder="First Name" required/>
<input type="text" name="last_name" placeholder="Last Name" required/>
<input type="radio" name="type" value="0">Owner
<input type="radio" name="type" value="1">Employee
<input type="radio" name="type" value="2">Artist
<input type="radio" name="type" value="3">Venue
<input type="radio" name="type" value="4">Fan<br />
<input type="hidden" name="fname" value="add_user" />
<input type="submit" class="button" value="Add" />
</form>
Also I apologize if some formatting may be off with my questions. First time posting and getting used to the site.