Often if the problem is a transient one it is a network problem. Issues in code tend to cause a script to fail all the time or not at all. Read / write permissions on data sources or written files (such as QVDs) can also cause problems.
You don't say what your data source is - this may help to pin-point the issue.
The other thing to do is turn on more verbose logging. Do this by opening the document in QlikView desktop and selecting Settings / Document Properties / General. On the right hand side you should see 'Generate logfile'. Select this and save the file back to the server. The next time the document is refreshed a more detailed logfile will be written. This should help identify the problem.
Post the relevant parts of the log file here if you are still having an issue.
I am unsure of the data source - you state both Oracle and QVD? Is it that you have one process creating the QVD from Oracle and then another reading from the QVD? If so you could have an issue if these overlap and the second process tries to open the file when it is still being written.
If things are reliant on the timings of loads being consistent then that could cause intermittent problems. If you do have dependant tasks ensure that they are set to run on completion of each other.
Also, if you are generating QVDs from a remote source it would make more sense to write the QVDs to a secure location on the QlikView server than to write them back to the network location.
Apologies if I have gone off on the wrong tack here...
The issue is with connectivity. Since if it is getting failed . I am creating the connect statement in the QVW file and the reload is working fine. But i am looking for the permanent solution. What is causing the connectivity to data source termination. is there any settings at QVW file or it is the problem with oracle....