Really strange problem with NAT router
I'm running a Slackware 10.2 as a NAT router, with latest kernel 2.4.32. The problem I'm suffering from is strange enough to sound but it is true! After some days of operation the router refuses to make new Internet connections - but which is curious not from all the machines that are behind the NAT, but instead from just let's say from 2-3 machines. At this moment, I can ping from those problematic machines any address with no problem, also I can open and browse the FTP site on the router itself successfully. And of course from the rest of the machines the Internet and NAT-ting is working perfectly.
Here are the simptons once again in brief in case I was not clear enough:
1. A network of 30 PCs with a Slackware router, NAT.
2. At some moment for 2-3 PCs (IPs) it becomes impossible to make new and any connections to Internet services: HTTP, FTP, ICQ, Skype, etc.
3. From those 2-3 PCs (IPs) it is still possible to ping and receive answer from any Internet site/address!!!
4. From those 2-3 PCs (IPs) I can access and use the Web/FTP service on the Linux router itself with no problems!!!
5. From the other 28 PCs (IPs) Internet access is still working perfectly.
6. The problem is fixed after rebooting the Linux - but regularly repeats itself again after some period of time.
Please, somebody shed some light how to diagnose this strange and absolutley unlogical situation and problem !
Just an additional note: this is a newly setup router which replaced a Redhat 9 Linux machine - when I return the Redhat machine in place the problem does not appear.