The tool PsExec is not designed to work with redirected input and output streams. This tool is part of the SysInternals PSTool toolkit: http://technet.microsoft.com/en-us/sysinternals/bb897553.aspx.
The command RunCommand by default redirects all output to fill the variables _stdout and _stderr. This redirection has been observed to hang current releases of the tool PsExec. To avoid this problem use the parameter CollectOutput (False) to disable redirection when calling PsExec.