Late season basil is bitter. Sad, but summer's ending. Your pesto should taste good immediately. I've heard theories about food processors making pesto bitter but I've been making it all summer in my cuisinart with no problem. On the other hand, last year, when I made side dishes for about 100 at a wedding (including a pasta dish), a relative of the groom provided pesto from her late garden. It was inedible, so I quietly subbed out a good store brand. I'm reasonably certain it was the age of the basil.