Error 500 only on permissions page I know that there are a lot of topic concerning the famous error 500 but i didn't find my answer in one of them.

Actually i have an error 500 when i load admin/people/permissions or when i'am trying to save new permissions. It's not all the time but i will say that it happens 80% of the time.
This permission page is very slow to load and when i haven't this error, it takes my navigator between 30s to 1m to display it.

I used google developper tools for this page and the result of the audit was:

36 requests  ❘  138.40KB transferred  ❘  27.57s (onload: 27.27s, DOMContentLoaded: 25.58s)

I edit my post and follow the trick that gave me H.NiK and i have impressive result for now

38 requests  ❘  17.09KB transferred  ❘  2.34s (onload: 1.76s, DOMContentLoaded: 1.42s

I have a lot of fields on the permission page more than 150 i think (because of the module fields permissions).

I watch the log of my server and i just have the same information: 

Premature end of script headers: index.php, referer: http://blabla.com/admin/people/permissions

I have 25 lines of this today on this page.

Have you any idea why i have this error only appears on this page?And maybe how i can track this error and resolve my problem? Is it possible that a module or a js file trigger this error? 

ps: i'm on a shared hosting, so i can't touch any configuration on my server (php.ini,my.ini)

Thanks,
Because you are on shared hosting the PHP process may not be given enough memory.  If you have a complex site or page the drupal bootstap may run out of memory and fail. I have had this problem trying to run a heavy site (one of the more Drupal complex distributions) on a shared host.  I would take a quick peak at how much memory is given to a PHP process (and Drupal) by visiting admin/reports/status and look for the "PHP Memory Limit".  I typically use 128M on a complex site.

You said also you can't edit you php.ini file - are you certain?  Many shared hosts allow you to to override the global php.ini file with one in your account.  Most will have a wiki explaining how configure such overrides.  If not you may need to move to a different host.