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Update your video adapter driver. If you don't have the disk driver, find out the make and download it from the manufacturer's web site. Or you can call the manufacturer of your PC and find out how to get the updated driver informing them that you're running Windows 98.

Enjoy!

Software DVD Players are very sensitive to CPU utilitization. Be sure you have done a couple things.

1.) Enable DMA for your DVD Drive. Goto to the Control Panel -> System Properties -> Device Manager. Goto the properties of the DVD Drive and check the enable DMA box. This is usually the biggest killer of DVD play back performance.

2.) There are superior software DVD players out there performance wise. Give WinDVD 2000 a try if you get a chance.

3.) See if there are any special Intel AGP Drivers out there. Sometimes call mini-port drivers, these may allow your AGP bus to perform better.

4.) Check your resolution, I would recommend 800x600 @ 16bit BPP for DVD play back. Anything higher and you're putting too much stress on a mediocre video card or the CPU.

Make sure that the Intel INF update has been installed. Right click on My Computer and go to properties. Click on device manager and go the system devices. If it says "PCI BRIDGE" in there, the update has not been loaded.

The INF update gives windows proper access to DMA calls, this I expect being your problem. Also make sure that the DVD in on is own channel, not slaved to the harddrive like some manuafacturers have a tendency to do!

If its Jerky then its from New Jersy.

I was going to suggest getting DirectX 7 but I guess you already did that.
Does this happen only with one DVD cd?
if it does then try another DVD cd
If it happens to all of them then I would go into the dvd players properties and adjust the sample play back rate. it may be called something difrent on yours but there is a properties box that should have the tools to fix it..
MATT

Your SIS 8MB AGP may not be enough to supply the graphic bandwidth needed to play your DVD picture unless you use a hardware decoder.
Install the latest SIS display driver if possible.
Why don't you try to use the latest graphic card such as GE-Force or GEForce 2. By that way you can use it to play games and it doesn't consume your proccesor speed much because it is the first Graphic Processor Unit.
If you are tight on budget, try to increase your display memory(if onboard) or increase your AGP Memory bandwidth equal to your system memory.
Hope these helps.