I think some of what people are hearing is the shift between words that you pronounce with a Russian accent and words where you are really hiding your Russian accent. It doesn't seem like you are switching between different regional American accents, but rather the foreign accent sometimes slips through and makes the sentence sound odd. I know you want to learn how to speak without a foreign accent, but I wonder if speaking in a relaxed way with a foreign accent is really so bad? Russian accents sound really cool, no need to be self conscious ... you will improve over time. Also, working on where you place emphasis in a sentence and the rhythm of the sentence will be very high yield. For example, your pronunciation in the sentence "I will not fail" is great, and the sentence only sounds strange because you place emphasis on "I" instead of "not". I am curious about how the paragraph would sound if read in a more relaxed way but with more natural emphasis on certain words.