So far in my view there is no exact solution for this problem. You need to restart gns3, but there are some tips that are very useful to resolving this.(With SecureCRT may get "Connection Refused by Remote Host" message instead of connection loss.)
- Add more hypervisors from "IOS images and hypervisors" when you are using more than 6 or 7 routers in gns3 topology.
- Try latest version of GNS3.
- On windows 7 run GNS3 as administrator in XP compatibility mode.
- Don’t save the gns3 topology while router executing the commands "write or copy run start".
- Try other IOS series instead of c2600, you can use 3700 or 7200 series.
Hope this will be informative for you.