Well, that's kind of to be expected - the rendering process is 100% the CPU crunching along; the soundcard doesn't play a part in it. Hard to tell what the actual problem is, but taking out the new card kind of proves my point here.
Check and see what ASIO or DirectSound devices FL is currently using. Maybe the one supplied with your new card isn't up to speed. If you have more than one choice, try the others and see which works best. (Settings: Audio)
Also check your buffer. Sounds like you need to increase it.
I had similar problems with a sound card change before in FL. Those two things solved the problem