I faced the following issue while working with SharePoint 2010
"The sandboxed code execution request was refused because the Sandboxed Code Host Service was too busy to handle the request”
Solution:
1) Open "Hosts" file from the following location:
C:\Windows\System32\drivers\ etc
2) Make this entry
127.0.0.1 crl.microsoft.com
4) Restart the
"SharePoint 2010 User Code Host" service on all these servers
3) IIS Reset
For other alternatives, please refer this blog.