I just wanted to let all you very helpful folks that my recently acquired sigma 120-300 did in fact have a mechanical problem with the lens and it wasn't me. 
I just got the lens back from sigma. They had to replace the HSM motor, which, I understand, drives the automatic focus. They also adjusted the focus. I was really lucky, inasmuch as the lens is still under warranty and Sigma honored the warranty even though I wasn't the original owner. Sigma service is also very honest. Naturally because I bought the lens used, I didn't think it was still under warranty, so on their checksheet, I said it wasn't under warranty. But they checked the serial number, and did the work under the warranty.
I will be testing it thoroughly at tomorrow's baseball game - this game will be of my southpaw - who's 13 and plays in a travel league. Look for some further pictures and posts from me tomorrow.
I feel very relieved knowing it wasn't me - I thought initially I was going bananas.
