WordPress MU (WPMU) / Multi Site – Sometimes images isn’t showing

I’m still new here – but I hope someone is willing and able to help me with my issue:

We recently merged our regular old school WordPress blogs into 1 WP MU/multisite installation.

But this came at a terrible cost.

I have 5 sites in my installation right now – and this is how I see them in my WPMU installation (for later reference):

Primary site:
site1.com

Secondary sites with own domain: (parked domain pointing at same server)
site2.com
site3.com
site4.com

Secondary site without own domain: (primary domain with subdirectory)
site1.com/site4com

All images IS WORKING on my primary site (site1.com).

On my secondary sites (with or without own domain), my images is sometimes working and sometimes not. I’ll try to explain the pattern:

In the morning everything is typically working. It “feels” like the number of visitors and how much they “stress” the system will provoke image load errors. When “it’s not working”, usually half the images is loaded correctly and the other half isn’t. Selecting one of the images not loaded and pasting the URL to a new pane/window and loading it alone ALWAYS works – no matter how quick and for how long I press F5 to stress the server.

So from above description I will try to conclude this error is related to how WP MU handles the secondary sites images. I think there is some “server forward”/”url resolve” logic different from WP MU and regular WP (I assume primary site works just like a regular blog).

So if someone could either:

• Help me solve this issue in total
• or help me identify the difference between primary/secondary url resolve

I would be so grateful !!

Thank you for reading this far no matter what!!

EDIT:
Just found a post on wordpress.org with the same issue – but no solution:
http://wordpress.org/support/topic/subdirectory-images-randomly-not-loading