The first thing I think of is, do these "couple" of users have the same browser in common? Do they have same issues using other browsers? Is it really only a couple of people? Do they use the same style?
The second thing I think of is, does this still happen with hooks disabled? Try it by going AdminCP>Options>Plugin/Hook System and disable hooks there, then have them test.
The third thing I thought of is, how heavy is his pageload? So I checked it and the results are not good:
You have multiple 404 errors from either missing or misnamed files, terrible first byte time and overall lethargic loading. There's alot of js involved here, alot of which isn't necessary - example: Do you really need visitors to see the chat box and the top ten stats?
70% of bytes loaded is from images... 14% from js.
Many of the js heavy mods can interfere with the wysiwyg editors, can also still be trying to load stuff on browsers while people are typing - this can be why it doesn't seem to "take" when they type. Checking this behavior out while hooks disabled is a must.
Yeerp..this happens on my site from time to time and I haven't figured out what mod is causing it ..very rare but it does happen ...