Unfortuantely, thats not a "crash", it wouldnt be easy to diagnose either, have you checked your server logs while this is going on?

server logs
apache logs
etc

This could even be a simple BW issue, FW issue etc

When it does happen, are you able to replicate it a few times in a row, or does it happen sporadically?

1- my host is following with server logs, he needs to catch the logs during the downtime, and it is a little bit hard
2- I happens sporadically but only when there are a lot of visitors online, e.g when 10k online downtime, 11k no downtime, 9k downtime...

I was hoping you could reproduce it multiple times in a row, but ohh well. Glad to hear your host is looking into it, will be interesting to see what they say.

When it comes back, are all the users still shown as online or does it appear that the users start showing at a low number and increase as time goes on? It could be that mysql is restarting if that is the case. So, check your mysql logs and see what is in there.