Hi,
It may be that SSH sessions are overloading the CPU and causing it to crash.
Telnet to it and watch the CPU for a period of time. Than open a few SSH sessions and watch the CPU again.
Couple of suggestions you can try:
1-Delete the SSH and than load a new one. If this doesn't help,
2-Try a different version of IOS and if this does not help, than you may have hardware issue.
HTH