Server 2008 x64
Xenapp 5

I have a citrix server that randomly grinds to a halt. On further inspection the logonUI process is taking up all the RAM. I end the process on the console and all is fine.

I had to rebuild the server from scratch anyway so was hoping that would sort the problem however it is still present

I have uninstalled sophos AV entirely (but did do a full system scan before doing so incase the logonui exectuable had been hijacked by a virus)

Does anyone know why this process might randomly try hijacking all 16GB of RAM? Im pretty sure it's not virus related.