Always "file changed on disk..."

[Peter Woo]

Under cygwin emacs on my PC, when connected to a NT ftp server (method
ftp), after 1st save of an edited file, subsequent saves (within a few
minutes interval) will alway make emacs returns the msg:

"foo.txt changed on disk; really edit the buffer?"

Have tried with the various backup options (on and off) but problem
still persists.  The more annoying thing is that after forcing for a
save, close buffer and re-open (if within a few minutes interval), the
old file content will be presented.

That NT ftp server's clock is kind of 5 minutes slower than my PC
clock.  Not sure whether related.

This can be reproduced on both tramp 2.1.2 and 2.1.3.

Please help.

Many thks,
P.