Ken, I can certainly understand your frustration. Unfortunately, we are somewhat at a loss trying to diagnose these issues without a lot more detailed information.
I also can understand how it might seem that the issue is "common" since you have it on 3 different machines, and have apparently seen it referenced elsewhere (though you didn't provide any links, and I haven't seen similar issues reported in reviews I've read). I use MC on a wide variety of different machines, and I have only ever seen a similar issue once (and it turned out to be related to a network hardware issue). One problem with saying "I see the issue on all my machines" is that most people configure all of their machines with similar software.... So if you have a "problem application" running on one machine, there is a good chance you might have the same problem with every computer you try it with! Rest assured, the vast majority of users are NOT experiencing these types of issues. We have seen sporadic reports, and most can be solved by the issues and tips Jim linked to.
Here are some things that we would need to help diagnose these types of issues:
1. System info. Contents of Help --> System Info.
2. File types that cause issues, including format (and codec if appropriate), bitrate, and file size. Similarly, are there any file types that do NOT cause the issue?
3. File location. Drive type, connection method (SATA, ATA, Firewire, USB, Network, etc).
4. If the issue occurs on network or external drives, does it also occur with files on a fast, local drive?
5. If the issue occurs on network drives only, all relevant network details (router, network cards, firewalls, cables).
6. What steps have you taken to ensure hardware and software stability and reliability? Memtest86 results? Prime95 results? Network testing?
7. Anti-Virus software installed, Disk Utility software installed (particularly encrypting systems and "on-the-fly" defrag systems), Anti-Spyware software installed, and other similar "resident" applications.
8. Any pattern to the issues? Do you see CPU usage spike (via Task Manager or Process Explorer) when the stuttering occurs? If so, what process is sucking up the cycles (MC or System or something else)?
9. Does disabling Podcast auto-downloading and/or disabling Auto-Import have any impact?
10. Does manually building Thumbnails overnight and then attempting playback ease the issue or have any impact?
11. Have you attempted to ensure that you are using the latest available drivers for your Soundcard? Have you tried the different Sound output methods (Wave Out vs. Directsound, etc)?
12. Is anything active in DSP Studio?
13. Any and all plugins that are installed into MC?