Two sites, same everything. Once fast, one slow

I have two sites, both same version of WP, same plugins on both, and both hosted by GoDaddy.

One runs very nicely. The other is having performance issues, to the point of my utter frustration. Most of the time, in either going to the site, or moving through the admin pages, it times out, putting error messages on the page.

The obvious answer is that the servers that they are running on are different servers. Godaddy is been somewhat of a help, but this is still an issue.

Part of the issue too is that this is sporadic. Some times it runs great, but the off times seem to be the rule rather than the exception.

Any thoughts on some type of cache program, or having it moved to a different server.

Thanks