Random sites, randomly trying to push POS traffic to our firewall

Scenario: we have a site that has a POS computer. It uses a program to complete transactions and sends it to a server internally in our network. We obviously use this for keeping track of sales and reporting, etc.

Randomly, the program goes down. Looking at our logs, our firewall is blocking the conversation between the POS and the server. The problem is that the conversation between the two should never be hitting the firewall to begin with.

This has happened 5 or 6 times in the last few months, and the only solution we’ve been able to remedy it with has been to change the IP address of the POS computer.

We’ve spoken to the people who make the program multiple times as well, and they’ve said repeatedly they don’t see anything wrong with the server as far as they can tell.

I’m just wondering if there’s something I’m missing here on a networking side of things. I can’t think of a reason for why seemingly random POS computers are trying to route through our firewall for an internal conversation.