I'm not sure the answer is to spend a fortune on another product - but that's your decision to make. I'll bet there are free alternatives out there if you search long and hard enough with the right criteria. But that's not really answering your question, so let's see if we can do that without spending money.
Check out the following article with a free download to customize speeds (and other Movie Maker add-ons
Blaine's Movie Maker Blog: WMM-6.0 - just skip the first one):
Blaine's Movie Maker Blog: Custom Speed Effects for Movie Maker (XP & Vista). I don't know if this will help, but it might and is worth a try (you can always uninstall it if you don't like it).
Do not consider the free Windows Essentials 2011
Windows Live Essentials 2011 which includes a new and improved version of Movie Maker. Unfortunately, it does not include speeding up or slowing down
at all - so it probably won't suit your needs if that's what you're trying to achieve.
I've also heard that different file formats work better at this than others - so see if you can covert the file to something else (AVI, MPEG, WMS, or whatever) and see if the change helps with the choppiness. I saw one thread where someone asked the exact same question as you and the response from the Moderator was "what file type are you using?" Unfortunately, that's where the thread ended. I've also seen where if the movie is played back from a second drive as opposed to the system drive, this has been known to happen - if that's the case, try saving it to the system drive and see if it works better.
You may want to check out the following site where they may already have answers to your question (or you can join and post a question and see if they offer any further help):
Movie Maker Forums.
Another possibility is that it may be an issue with your graphics card or its drivers. Since we don't want to get another graphics card (and spend money), let's at least update the drivers to the most recent and see if that helps.
Proceed as follows to get the drivers: http://pcsupport.about.com/od/driverssupport/ht/driverdlmfgr.htm. Once you have the drivers, you can install them through Device Manager as follows: http://www.vistax64.com/tutorials/193584-device-manager-install-driver.html.
It could also be memory where it can't keep up with the increased speed and thus acts like a buffer in online videos. You can test your RAM at http://www.memtest.org/. If it fails, test each chip individually to find those that are bad and replace them. If it passes, RAM is probably OK.
Tell me the filetypes you are using. Are you playing these from the hard drive? If so, try burning one to a CD and see if you have the same problem.
If that doesn't work, try a clean boot http://support.microsoft.com/kb/929135. If the problem goes away then it's just a matter of tracking down the culprit causing the problem. Follow the procedures in the article. Once found, delete, remove, deactivate, or uninstall it. Once done be sure to reset Vista back to normal status as explained in the procedures. If the problem occurs in clean mode then just restore the system to normal status and reboot - this solution is not going to work.
That's enough for now. Let's see how things go with these suggestions and alternatives.
I hope this helps. If not, post back and we'll do some further research.
Good luck!