SSH to the server as root and do the below one by one to see if the server is overloaded
and if it is causing it,

top -c   ( load in the vps and what is causing it )

netstat -anp |grep ‘tcp\|udp’ | awk ‘{print $5}’ | cut -d: -f1 | sort | uniq -c | sort -n

netstat -plan|grep :80|awk {‘print $5’}|cut -d: -f 1|sort|uniq -c|sort -nk 1    (
number of connections and the ips making )

tcpdump -i venet0 port 80   ( this one gives you raw details for connections )

tail -f /var/www/vhosts/     ( shows details of what exctly
is being acessed. )