Wierd Flavors

I have brewed several batches so far and I notice several interesting things about my brews. First is that several glasses that I pour from my keg will taste amazing and then I will get one that tastes terrible (bad after taste and strong bitterness not characteristic of the style made) and have to struggle through to finish it. Does anyone else get this or is it just me? and what could be the source of the problems? Thanks

Wow...not being a kegger, I'm not sure, someone may come along in a bit...But havin said that the thing that occurs to ME at least is a sanitization issue. Your beer is over the time it's in the keg/lines, it is getting infected with something...I would look at your cleaning and sanitization process....it reminds me of the bars that don't flush their lines repeatedly and some of their bers taste like crap...