As this is working intermittently I would first verify that the correct transport (and only those) are installed for the SAP version and connector you are using. It's possible you could be experiencing intermittent connectivity due to transport conflict. Please verify this per the help pages:
Otherwise, are these servers in a local topology? Same datacenter? What type of link? If you suspect the connection is getting interrupted, you may want to try to segment the data into smaller sets and/or reload the data from QVDs.
I hope that helps,
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