Difficulty with A/V in MKV format.

I'm having some trouble with playing MKV files; the audio plays fine, but the video lags behind pretty quickly. I've only had the problem with some series, though; Higurashi/Rei and Hidamari/Hoshimitsu lag pretty badly, for example, but Natsu no Sora played without incident from start to finish.

I was using the last stable CCCP, and tried switching H.264 to MT to no avail, so I downloaded the latest beta and got no farther. My MPCHC version is 1.3.2212.0 (the one that came with CCCP), and I haven't changed any settings.

My laptop specs -- and they're not great, since this is an old machine:

Dell Inspiron 8200
Intel Pentium 4 Mobile CPU 1.60GHz
1 GB RAM
Windows XP (SP3)

Insurgent Render Test shows the following:

Filter 0: Default DirectSound Device
Filename: C:\WINDOWS\System32\quartz.dll
Filter CRC: C4A59DCC
Date: 2010-02-05 | 14:27:45
Filter 1: Video Renderer
Filename: C:\WINDOWS\System32\quartz.dll
Filter CRC: C4A59DCC
Date: 2010-02-05 | 14:27:45
Filter 2: ffdshow Audio Decoder
Filename: C:\Program Files\Combined Community Codec Pack\Filters\FFDShow\ffdshow.ax
Filter CRC: 46F91329
Date: 2010-08-07 | 08:49:16
Filter 3: DirectVobSub (auto-loading version)
Filename: C:\Program Files\Combined Community Codec Pack\Filters\VSFilter.dll
Filter CRC: FA0DE7CF
Date: 2010-08-07 | 09:39:30
Filter 4: ffdshow Video Decoder
Filename: C:\Program Files\Combined Community Codec Pack\Filters\FFDShow\ffdshow.ax
Filter CRC: 46F91329
Date: 2010-08-07 | 08:49:16
Filter 5: E:\TV Dump\Anime\Higurashi-Rei\[WinD]_Higurashi_no_Naku_Koro_ni_Rei_-_01_[BD-720p][8E5CA65F].mkv
Filename: C:\Program Files\Combined Community Codec Pack\Filters\Haali\splitter.ax
Filter CRC: B87FE393
Date: 2010-05-19 | 13:59:52