In this case:
1. Check if xCache has enough memory (install/direct to its panel)
2. Try disabling all plugins and leave just W3TC. If it works fine, then other plugin(s) are causing the issue. Since W3TC is a very wide spread add-on, I guess the problem isn't with W3TC
3. Try to check your server configuration
4. Try to setup a testing area at a different hosting company. Maybe your host is the problem.