Last week, I decided to allow Quicktime to update itself (I had version 7.2 with 7.2's security update). Its updater volunteered that version 7.6.6 was available, so I said, "OK" and did it. A few days later, I decided to watch a Tekzilla episode I'd downloaded some months ago and forgot to watch. I couldn't watch it smoothly. Every few seconds to sometimes as much as 5 minutes, it would just halt or lock and replay the last second or so. I'd downloaded the episode in HD format, so I tried lesser formats to see if that was the problem. All versions of any episode I tried locked, stuttered, paused, lost voice synch, etc. at random but short time intervals.
After downloading version 7.2, I uninstalled Quicktime 7.6.6. Then, installing QT 7.2, watched the same Tekzilla episode that had failed. All was right again! It was 7.6.6. Now, I have an old system, a 2002 Dell Dimension with 2.8GHz P4, Intel MB, 1G of memory, Nvidia GeForce 7600, but getting this kind of problem from Quicktime...sucked.
Steve Jobs, you're working to earn the Linden prize?
If you've been having a similar problem, you can get older versions of Quicktime from Apple here