There is no known instability in the ftp client. The web server (Google’s open source mongoose) does have occaisional problems but it is not know why – thoughts are the Foxsat just runs out of steam.
You can see the status and control mongoose and other services via telnet using the service command. For example:
service
service start mongoose
Advice is wait unil mongoose fully finishes a task before starting another.
If you have ftp problems I would suspect other causes such as network.