I noticed that access to certain posts in Forum is becoming slow.

http://www.panfluteforum.org >FORUM

Navigating through menues and categories is no problem.
Click and reaction in less than 1 second.

Clicking on a single post which has INLINE PICTURES lasts
up to 40 seconds or more to get there.

Posts without an INLINE PIC react instantly.

What can be done to handle this?

(Forum has a total of about 1250 posts).

Strange. It's back fast again. It seemd that problem was only temporary. I'm wondering what could be the reason for that?

Just server load probably - I'd check with your webhost to make sure nothing's overload.