I've just had this happen to me and it was Charles, Spryly, and Tig's fault and they all have randimg avatars. I saw my reply box refresh the text and then I refreshed and it loaded fine, I hit preview post this time and it lagged so I opened my switcher while it was still loading the page and Charles and Spryly didn't have avatars loaded in, Tig's avatar in the post preview I then noticed was also missing. I know you said you're not using Randimg but if this is still happening, try to check the favicon before starting your post to see if it's still loading and if it is, check your switcher, scroll through the cbox (if it's loaded into the page), just see if you can spot any missing images before the page finishes loading. It might finish loading the problematic image(s) or it might give up and leave a broken image/no image.

I think possibly one of the confusing factors here may be cache. If your browser has cached a slow loading image then you'll get some quick page loads until it tries to recache that image and then it'll lag again.