Webpage image sequencer begins to lag after time

http://www.henryneilson.com/HyBird3/Test.html

I have this image sequencing landing page, that I'm working on^

The sequencer pre-loads about 186 images and shows them sequentially when the document is scrolled.

After all the images are loaded, everything runs smoothly,however after perhaps 10-20 seconds the whole thing starts to lag. It seems that the script is keeping up with changing the image number, but the image won't visually update in the fine increments it should. I just skips frames horribly.

I know I should compress the images more but if it works initially I don't see how it "develops" this lag?