I have seen numerous posts about this, but I just wanted to see if I was
right in diagnosing this issue
My specs are listed at the bottom of this post.

In Doom3 (the demo), I get little 'pauses' quite often, more so when opening
doors etc but also just when moving along with no real change (that I can
percieve). In Half Life 2 (full game) I was getting the same thing, but
these reduced quite a bit when I changed the sound from 5.1 to 2 speaker and
from High to Medium.

I have Onboard sound (Realtek AC'97 - latest drivers), and I have read some
posts that say that this sucks CPU cycles and that HL2 and Doom3 need the
CPU a lot.

Could this be the problem or could it be something else?
I choose the optimal settings for HL2 (full game) and Doom3 (demo) -
basically medium quality, no AA and a res of 1024x768. Sound settings for
both are 2 speaker.

Specs:
Intel Pentium 4 3.0GHz (1MB cache, 800FSB) with HT (Prescott)
MSI 865PE NEO-2 P MS-6728 Ver 2 (No D-bracket) 3.50 AMI BIOS
Nanya 2 x 256MB dual channel DDR RAM (PC3200) 400MHz
256 Mb ABIT 9600XT (Catalyst 5.1)
FSP Group FSP300-60PN 300W +3.3@28A, +5V@30A, +12V@18A,
, ,
Western Digital 160GB 7200rpm hard drive with 8MB buffer
Onboard Realtek AC'97 Audio
Sony DVD-ROM drive (16x)
Sony DW-D22A Dual-Layer IDE DVD+/-RW (black)
Speedtouch 330 USB DSL modem
Windows XP Pro SP2
memtest86 1.27: 7.5 hour all tests no errors
Motherboard Monitor Voltages: , ,
3DMark2001: 13200, 3DMark2003: 3800, 3DMark2005: 1600