連線斷線是網路卡造成的嗎？

Idiot

Hmm...  I think there are not enough information for people to figure out what is wrong.

For example, are both computers at the same location?  Are you using the same cable for both computer to test?  Same software to connect?  Same settings?  (Dynamic/static IP?)

If both computer are at the same location, using same cable, software, settings for test, then probably it is true.  Network card is very cheap these days, you can probably get one and try to see if that is the problem.