unable to access public ip

unable to access public ip

my server is configured to have a public ip on one network card, and a
private ip on the other network card... the server was running fine after
some time... suddenly i'm unable to access my server from outside my isp
network...
at first i thought it's because i recently changed my ip address, until
the same thing happen to 3 other servers (that i'm in charged to) on
totally different network... one is running with the same isp (same
network as my previous server), one is running with different isp (not
even in the same country), and the other one is running with different isp
(same country)...
the conditions i checked :
from outside the server isp network:
able to ping to the server
traceroute to the server has no problem
unable to access the webserver
unable to access the ssh
unable to access the sftp
from inside the server isp network (using public ip):
able to ping to the server
traceroute to the server has no problem
able to access the webserver
able to access the ssh
able to access the sftp
there's no firewall set on the servers... is there anything i should
check? i'm also asking to my isp as i put this question... this also
happens only on occasion...