Thank you so much for the elaborated analysis. I'll follow your advice to go to the bike shop where I bought it to have it checked out. However I'll have to wait another month or so when I'm less busy and have time to go (it's not nearby and out of my way of daily commuting). Today I spent hours and gave the bike a thorough cleaning, including the chains and derailleurs, and re-adjusted the limit screws to the best I could (the reason I had re-adjusted the limit screws was that I stupidly tightened all of them the other day on a whim, totally ignorant of their functions--now you have idea whom you are talking to, heehee). I'll see on my next ride if the situation improves or not. Hopefully the problems that you summarized above will not cause serious damage in the following month if I pay attention and minimize gear shift before I take it to the bike shop (?).