the Board does not accept signal from COM port

Board receives signal from com port through times ,then work ,then not work.

Bad cable.
Bad contact; make sure cables is plugged in properly.

Is this during upload or when your sketch is running? If the latter, please provide sketch (and use code tags as described in the various "how to use this forum" stickies).

Which board?
Which operating system?
Just in case, which version of the IDE?