Java process demoted to single CPU

please help on this issue.

I take care of a production enterprise application , it is a java process quite complex that is a middleware application.
The process runs 24 hours a day. Usage cpu is 60% as mean value all day.
The process have some burst of cpu usage at 100% usage in the afternoon.
The process runs in a WMWARE virtual Server:
the server is configured 2 CPU, 4GB ram;
the server is installed as a Linux Kernel 2.6.18-274.12.1.el5.
Java version installed is 1.6.0_16;
In a unpredictable moment, mean 2 times a week, the processes is misteriously demoted to Single CPU,
so it takes maximum 50% CPU and application becomes unavailable because not capable to serve all requests.
Does anybody knows if there is an "auto demoting to single cpu" mechanism in Java ?
Or in Linux?
Or in Wmware?

thanks in advance

Alberto