My Captcha doesn't work every time

I have the following code:

<script type="text/javascript">
 image = "http://mysite.net/captchaimage.php" //name of the image
      function NewCaptcha() {
      tmp = new Date();
      tmp = "?"+tmp.getTime()
      document.images["captcha"].src = image+tmp 
}
</script>

<img src="http://mysite.net/captchaimage.php" alt="CAPTCHA" name="captcha" />
<br /><div class="attendst">
Too obscure? <a href="javascript:NewCaptcha()">Reload</a>.</div>			<br />

			<label for="captchafield">Enter the text from the image above.</label><br />
			<input type="text" name="captcha" id="captchafield" />

PHP:

Which is connected to this code:

function createCAPTCHA($ref) {
	//Create image
	$captcha = imagecreatefrompng("./captcha.png");

	//Define colours
	$black = imagecolorallocate($captcha, 0, 0, 0);
	$line = imagecolorallocate($captcha,0,0,0);

	//Start the session
	session_start();

	//Use md5 to generate a random string
	$md5 = md5(microtime() * mktime());

	//We dont need a 32 character long string so we trim it down to 5

	$string = substr($md5,0,5);

	//Add string to image
	imagestring($captcha, 5, rand(30,90), rand(0,60), $string, $black);

	//Draw lines on image
	imageline($captcha,rand(30,70),0,rand(0,155),79,$line);
	imageline($captcha,rand(30,100),0,rand(0,155),79,$line);


	//Store string in session	
	$_SESSION['captchakey'] = md5($string);
	$_SESSION['ref'] = $ref;

	//Output the image
	
	header("Content-type: image/png");
	imagepng($captcha);

imagedestroy($captcha);

}

PHP:

This issue is, sometimes the captcha works and sometimes it doesn't. When it doesn't work it will just give the "incorrect captcha" error. What could be causing the code to not recognize the captcha only sometimes. Anyone have any suggestions?

Thanks!

 

Just as a test, save the captchkey in plain text in the session.. so no md5($string) but just = $string;

Then, in your validation function, output the input ($_POST['captchakey'] or whatever) and the $_SESSION['captchakey'] for debugging purposes.