SharePoint Designer created workflow will stop before the Send Email action. This may only happen for certain users, therefore, making the issue seem to happen intermittently.
This will show the workflow status as Stopped.
Send Email action has SharePoint group(s) and/or workflow lookups in the To / Cc line.
Make sure each SharePoint group has at least Read permissions for the site.
Additionally, that in the Group Settings area of the Group Page, under Who can view the membership of the group, Everyone is selected.
1. On SharePoint site, click Site Actions -> Site Settings -> Modify All Site Settings.
2. Under Users and Permissions heading, click People and groups.
3. Select appropriate group(s), click Settings -> Group Settings.
4. Under Group Settings area, select Everyone for Who can view the membership of the group, click OK.