Thumbnails randomly disappear

Hello,

I'm experiencing problem on my Gallery 2 site which can't display all the thumbnails of a page.
The site administrator thought it could be a memory issue, but even after increased the php memory limit from 64M up to 128M, the problem is still present.
When some thumbnails are not shown, you can make it appear by refreshing the page, but in this case, some other thumbnails could disappear.
Then the site administrator says that it's because it takes time to fetch images from the application.

Does anyone have had the same issue and how do you have got ride of it?

Thanks in advance,

Yuan

System information:
Gallery version = 2.2.6 core 1.2.0.8
PHP version = 5.2.14 cgi
Webserver = Apache
Database = mysqli 5.0.92-community-log, lock.system=flock
Toolkits = LinkItemToolkit, SquareThumb, Thumbnail, ImageMagick
Acceleration = partial/86400, partial/900
Operating system = Linux web6.cirtexhosting.com 2.6.18-294.26.1.el5.lve0.8.17 #1 SMP Wed Feb 16 16:49:30 EET 2011 x86_64
Default theme = carbon
gettext = enabled
Locale = en_US
Browser = Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3 QQDownload/1.7
Rows in GalleryAccessMap table = 21
Rows in GalleryAccessSubscriberMap table = 1711
Rows in GalleryUser table = 8
Rows in GalleryItem table = 1707
Rows in GalleryAlbumItem table = 101
Rows in GalleryCacheMap table = 7058