esx server "freezes"

I have an esx 4.0 server that "freezes" randomly.

When I look at performance I can see blank gaps in all the "switch to" areas.

Don't see any errors or warnings.

Server

Hp dl 380 g6

2 5520 procs

48gb ram

I have another one just like it but 36gb ram with no problems.

Both are running the same version esx.

Kinda looks like hardware, but I get no errors or warnings.

Thanks

Tom