Network loses connection to server but not internet connectivity

Hello, ive got a network with four computers running with Windows 7. All of them are connected to a switch. My problem is that the internal network connection is lost very often, although internet connectivity works ok. The problem usually happens while using a management software that runs with MySql db. This program is installed in the server. The support department of this software tell me their software works ok, the issue is caused by my network but I haven’t been able to figure out the problem and the network seems to work ok.

Any ideas?

Thank you.