First, please tell us (at a high level) everything you can about the way your splunk is set up. How many search heads, how many indexers and forwarders, are they clustered, any other high level notes about your architecture.
Also, please walk through, step by step, the steps that you are using to create the dashboard, and post them in the question. A detailed list of those steps may give us some insight that can help resolve the problem.
A saved search will expire and autodelete, but I can't find any indication that a dashboard should. If this is happening, then it's probable that some other administrative process is cleaning up your local files. Perhaps you are saving the dashboard to a local area that is being refreshed/overwritten by replication.
In any case, some hints about your organization's splunk setup are going to be needed to help you isolate the issue, I believe.
