Regular session table crashes, on a large multi domain, multi site Drupal set-up I have recently been assigned to a project that has regular session table crashes.

Drupal outputs an error like this one: 
"Warning: Table './[database]/sessions' is marked as crashed and should be repaired ..."

The problem with this site is that it started as a small drupal site and various companies have added (custom) modules to it. The site has also been ported to a number of different hosting providers with varying results. Currently it is being hosted at OVH on a shared hosting setup.

The site is rather large now with a constant base of 20 users inserting data simultaniously (on different accounts but through the drupal interface), mostly pictures through a multi upload module. At the same time there is a lot of traffic coming from anonymous users (who do not upload anything at this point).

I think the problem might be the shared hosting platform. I think it might not be able to cope with all of the different inserts and just crashes...
Normally I would see if I could cause the crash in some way, but I can't take the site down since it will result in a major financial loss to the company.

I've read some other posts here regarding a large session table where they mentioned the "ini_set('session.gc_maxlifetime',   200000);" line in the settings.php file. 
The "answer" was to reduce the amount of sessions to 900-1800 which would result in a smaller session table, but I don't know if this would solve the problem?

To make matters worse my client cannot give me any information on when the crashes occur, if they are perhaps caused by using a custom module, etc.
The site is running on Drupal 6.

I realize there is not a lot of useful information in my post and I apologize for that.
I am a new Drupal developer and I don't know how to address this problem.
(The developer who was handling this issue before I got here quit...)

Any help in this matter would be appreciated.

Thank you.
I would convert the user &amp; session tables to InnoDB. I would also check the memory limits for MySQL. It's not the most ideal tool, but if you're in a shared environment you can install a copy of Drupal Tweaks (http://drupal.org/project/drupal_tweaks) and see if you are able to alter any of the MySQL configuration directive such as memory limit.