MSSQL 2005 Linked Server to Progress v9.1C on Linux throught Merant 3.60 keeps failing

Hi,

I have added in MSSQL linked server to Progress v9.1C database using such commands:
EXEC sp_addlinkedserver
   @server = 'SAF',
   @srvproduct = '',
   @provider = 'MSDASQL',
   @datasrc = 'SaFmer'
EXEC master.dbo.sp_addlinkedsrvlogin @rmtsrvname=N'SAF',@useself=N'False',@locallogin=NULL,@rmtuser=N'root',@rmtpassword='root_pass'

Connection works fine, I can retrieve and update data.
We start running our application, written in VB.NET, which does all work using that linked server.
Everything works fine, for a while. Then in random time spans, I start getting error:

An error occurred while preparing the query "SELECT bla bla bla ..." for execution against OLE DB provider "MSDASQL" for linked server "SAF".
OLE DB provider "MSDASQL" for linked server "SAF" returned message "[MERANT][ODBC PROGRESS driver][PROGRESS]Server not reachable or possible RPC error"

After few minutes of these errors program "fixes itself", works few minutes fine and again goes back to error messages.

maybe anybody experienced this kind of behaviour and could recommend where start looking for problem or some kind of workaround?