Jerky stuttering video

[Kmilsark]

Greetings--I hope someone can help me with this; it's puzzled everyone I've talked to. For the past month or two, video performance on a couple of games we play has gotten very poor--instead of moving smoothly, things seem to start and stop. The games involved are Zuma Deluxe and Luxor. Luxor even stopped running altogether; just a black screen. Neither game is very graphically intense. Other games that are more demanding, like Prince of Persia Sands of Time and Indiana Jones & The Emperor's Tomb, seem to run fine. DVDs play fine. Specs are: MSI mobo (KT4AV), AMD Athlon XP 1800 chip, 768 MB RAM, Windows XP SP2, and an ATI Radeon 8500 AGP w/ 128 MB RAM. I swapped the ATI card for an nVidia GeForce 4, and the problem has eased somewhat, but is definitely still there. Device Mgr shows no hardware or memory conflicts. Has anyone else run into this, or have any suggestions on what to do? Thanks! Keith