Brian, the final syllable gets its stress only because of promotion. When we Americans say the word, the emphasis is very much on the second syllable, as you can listen to
here. There are many words, I have noticed, that Americans pronounce with a stress on the third-to-last syllable but Brits pronounce with a stress on the second to last syllable. It's very FRUStrating to us, and probably frusTRATing to you.