You could have a faulty switch or router or patch panel or cables or nic card(so hardware problem) check your hardware. It could be your not supplying your nic with enough power, power supply upgrade. You could have a setting in your nic properties enabled that says disconnect if idol for 30 minutes. If it is wireless your connection could be dropping because of other interference that occurs every 30 minutes. You might have a virus, or spy-ware. A script could be running in the back ground that continually disconnects and reconnects the connection every half hour.
Get your line tested, it should be a free service form your telecommunications provider, There could be a fault on your line. You also might have been hacked and someone, who periodically every half hour, has a program running that is sending a denial of service attack (DOS attack) to your PC.
Except for reinstalling the OS, I think you have ruled out the PC as being the problem (although replacing the NIC might possibly solve the problem). Next, try to test the modem you use to connect to your ISP. If it tests out fine, ask the phone company to check their equipment. I had a similar problem once which was traced down to a switching box in an industrial complex near here.
If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
Events
Event ID 26 "Warning" Intel(R) PRO/1000 CT Network Connection Link has been disconnected.
Event ID 27 "Information" Intel(R) PRO/1000 CT Network Connection Link has been established.
Any ideas
TIA
Rob