Citrix Xen is not a supported platform for PGP Universal Server 2.x, 3.0.x or 3.1.x. I have heard of some customers running in such an environment but do not know if they have seen the instability you report. It is worth noting that the 2.x series was based on fedora core and 3.x is CentOS based. See if you can find instructions on how to make those platforms happy in a Citrix Xen environment. I expect that following such instructions will help significantly with the instability.