WP 3.8.1 is causing multiple errors 500

Hi,

The upgrade to WP 3.8.1 has been causing a great number of 500 errors on a website i manage. The errors come & go, it’s like someone is switching the lights on and off. The website is quite a wide audience (4000 visitors per day / 20k pages). So these errors are very disruptive.

For the moment, i’ve done the following changes

– Increased MAX Memory to 128 MO
– Disactivated some non essential plugins and essential plugins that have numerous “broken” reports betwwen their latest version and WordPress 3.8.1 (W3 Total Cache or WooCommerce for example).

The number of errors has fallen significantly but not completely. I’m wondering what my next step should be knowing :

– The theme was developed for WP 2.X (2.8 i believe). Could this be an issue ?
– The website relies on quite à lot of plugins : A forum under Simple:Press, A FAQ plugin, Akismet, JetPack, Contact Form 7, DMSGuestbook (don’t know why this was chosen), Wysija and a few more. I realize this could be the problem but things were working fine until a few days ago. Any insights on how to manage this issue ?

Do you recommend i downgrade WP by installing an older version (3.7.1 or 3.8 for example) ? What precautions should it take if i go down that road ? The website database is pretty big (500 articles, 25K comments, 1000 forum threads for 5000 messages. WP export feature is giving me crap. I’ve done a copy of the database but is there anything else i should be doing ?

Thanks for your help !
Pierre