Testing image-rendering PHP script on shared hosting with JMeter

I'm testing two versions of the same script. Each script does the same operation but they differ in how they output the result. Each script:

1. Loads a local image using imagecreatefrompng()
2. Adds text to image using imagettftext()

The first script outputs the final image this way:

imagepng($img, $cacheFile, $quality);
imagedestroy($img);
$fp = fopen($cacheFile, 'rb');
header("Content-Type: image/png");
header("Cache-Control: no-cache, must-revalidate"); 
header("Pragma: no-cache");
header("Content-Length: " . filesize($cacheFile));
header('Content-Transfer-Encoding: binary');
ob_end_clean();
fpassthru($fp);

2nd script:

header('Content-Type: image/png');
imagepng($img, null, $quality); 
imagedestroy($img);

Both scripts work well when I call them directly in the browser and I get the expected result. But when I use JMeter for load testing of each one, I often get a non-descriptive 500 errors in response in each script.

The error_log file, which is normally created when there are errors in the script, is not present.

How can I troubleshoot the cause of the 500 error? Is there a way to know if it's the shared hosting cutting off resources and that's why scripts fail to execute or if it's something else? How can I optimize my scripts for best performance during a heavy load?