We had the same problems with Ntop (we monitor netflows, not local interfaces). There was a thread about a good netflow tool a week or two ago where someone suggested NfSen + nfdump. I set it up Thursday (took 30 mins or so) and it totally rocks. We've gone to production with it to monitor flows from around 50 devices. I highly recommend you take a look if you have serious needs to see what's really on the wire.