Strange IP ports behavior

[Massimo Villa]

Sorry for my description but I'm a Linux's novice. I have a pc Compaq DP133 MHz with kernel 2.2.19pre17 acting as mail server with PostFix. I don't know why in some day (not all) the different IP services (SMTP, POP, TELNET) are unavailable for the timeout period after that I 'm able to telnet or sending or receiving mail. The strange is that the day after it's all right and after 1 month or a week the problems appears again. The linux pc has a private IP (192.168.0 class C) natted to a public address. And I have the problem only when the others private IP try to connect with this machine for any IP services. If I put all the privates IP in the "hosts" file, I partially resolve the problem (SMTP service for example goes only after the 75/80 seconds of timeout). It's seems a problem of reverse lookup. I haven't any internal DNS server for private resolving addresses. My resolve.conf files has simply these two lines nameserver 212.31.224.2 nameserver 212.31.224.3 and I try to change the DNS addresses but the problem remains. There anybody who can tell me a possible solution or some tests to solve this problem?   Thanks