Your problem is almost certainly related to a lack of resources. Things to try:
1. Get a better idle-pc value. See The Illusive Art of Acquiring A Good Idle-PC Value
2. Try and use the same router model for all router instances, and the same image, and make sure Enable ghost IOS support
is enabled under Preferences->Dynamips
3. Try reducing the amount of RAM used by your router - although this may lead to stability problems.
4. Use a better PC or better OS - I find GNS3 works much better on OSX than an equivalent Windows PC
-a VMware image of Ubuntu with GNS3 and VPCS installed and a collection of exercises/labs