Jerky audio/video on new system??

We bought FS2004 at Xmas and installed it successfully on my computer ...
(Dell 4 1.8 GHz, 512 Ram, NVidea GeForce3 Ti 200 video card, new Microsoft
Force Feedback joystick, XP Professional).

So, we decided to upgrade my husband's computer so he could run it. We just
took delivery of a new Dell 8300 (Pentium 4, 2.8 Ghz, 512 Ram, 128 ATI
Radeon 9800 card, XP professional) this week. First thing I did after the
setup was to install FS2004 on it and transfer the joystick.

But ... on the new Dell, flight training centre sound is a bit jerky and
when opening/closing modules the video flashes multiple times. The older
Dell works better.

Any ideas? The system is only a week old! ( I tried changing the sound to
no acceleration on the Direct X console, but that made the sound worse, so I
restored it.)

Thanks