Let them clear their cookies.

We've had users clear their cookies, and check their privacy settings are default to Medium and we're still getting random logouts on 3.5.3. The plugins we have are:

• Archive Adsense 1.0
  Edit New Thread Button 1.1
  Extended Signature Limits 1.0.3
  Form Hack 3.3
  HELLCATs Realtime Page Compressor 1.24
  Moderator Stats 1.2
  Reported post creates new thread 2.01
  Security Fix for Issue in vBulletin 3.5.3
  SigmaChat 1.0.0 SigmaChat integration
  Strike Through Closed Threads 1.0.0

I can't see how any of these would affect it. Such a strange issue this, there are plenty of people reporting it across vB.com and .org but at the same time plenty of people without the problem. I wonder what the common link is?

Upgrade to 3.5.4
Reset your cookie domain/path to default.
Create a new Style with no Parent set, let your members use this new style, and see if they still have problems.
Try disabling the Page compressor hack.
Turn off GZip-compression in your vBulletin Options.
If still problems, disable all hacks.
Make sure your members who are reporting this are checking Remember Me box when logging in.
What is the setting for you Cookie timeout?

Perhaps it would be best to actually upgrade to the later versions and not just apply security fixes/patches? I havent personally encountered this error and i seem to be using some similar modifications to you too.

Thanks for this Marco, I have already tried resetting the cookie path. The timeout is the default (900 I think?). At the moment users are indeed using a child style, so I will give your suggestion a go. As for "Remember Me" I have that set to checked automatically. The problem was happening before Page compressor so I'm confident it's not that. Cheers.