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Sporadic FTP error when moving files from a PC to MVS

We keep receiving FTP error code 451 when transferring files to MVS from a PC. Sometime it will work, and it's inconsistent on when it fails. Any ideas?
This is probably the FTP server problem. What FTP server is installed on the PC side? One possible reason is from Microsoft:
When FTP clients initiate passive file transfer operations, the FTP server uses asynchronous network notification to wait for the actual client connection to a passive data socket. By default, the FTP server allows 60 passive file transfer operations to be queued simultaneously. Once the queue reaches 60, subsequent client requests receive error 451: Too many Passive Users. Client connections remain in this transient state for a short time, between issuing a file transfer command (e.g. RETR, STOR) and connecting the data channel to the passive socket. For this reason, this error can only occur in one of the following scenarios:

-- A DoS attack has issued concurrent file transfer commands without actually connecting to the data socket.
-- An extremely large number of concurrent users are transferring files in passive mode (which is possible on slow networks).

You may try to work around this problem using non-passive file transfer or changing a registry entry as described here.

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