When you try to edit your custom web part, if you get an error message 'Intermittent "HTTP 403 – Forbidden" error ".
This issue is due to permission missing in bin folder of your web application.
We should make sure that Users group has Read & Execute, List Folder Contents and Read permissions on the /BIN folder below C:\inetpub\wwwroot\wss\VirtualDirectories\Sitename:80
Steps:
1. Go to the /bin directory of your web application.
2. Right-click on the folder and click on Properties.
3. Go to Security tab and click on Edit.
4. Click on Add and add the local server group Authenticated Users or <SERVER NAME>\Users.
5. Select the Read & Execute, List Folder Contents and Read permissions .
Happy Share Pointing :-)