Starting problems

Hi - I have a 2004 Taurus SES 3.0L, 61,000 miles. Daily driver, battery replaced 4 years ago (Sears DieHard) but all else on the car is original. About a month ago I had a rough start - cranked over and over, finally started. I figured it had to do with the recent spring weather (condensation in fuel tank/lines?). It then started/ran fine for a week, then had the same problem except this time, it cranked over and over without starting until the battery died. I jumped it - started right up and ran fine. Took it to my mechanic and he found nothing wrong with anything. I then took the car to Sears and they checked the battery (that they sold me four years prior) - its CCAs were fine so no replacement was needed. So, I used the car for three weeks with no trouble (daily driver, so several starts per day), then the same problem came back - cranked with no start, until the battery was dead! I jumped it again and took it to a different mechanic and told him the story. They tried for two days and found nothing wrong with the car! Heh - I'm driving it around daily again, but I'm waiting to be stranded again and hoping to avoid it. The first mechanic said his best guess is that the battery has warped plates of some sort. The second mechanic had no guesses and hadn't seen this type of problem before. I have no engine lights coming on, and the mechanics don't think it is a fuel pump problem. Any ideas? Thanks in advance for your time and any help you can offer!