[jQuery] jquery.noConflict, $j is not a function in some case(Not always) on the same page

[jQuery] jquery.noConflict, $j is not a function in some case(Not always) on the same page

This is my site Mychico.com!
I intergate an Ajax IM into site, this IM use prototype! So, i must use jquery.noConflict
I don't know why sometimes, FF notice that $j is not a function!
it is not always happen! just in some cases!
Referesh about 10 times then 2 times hapen $j is not a function! And since it report $j is not a function! We only need clear cache then refresh page then this is not happen! But refresh more times, it report $j is not a function again!
It's so strange?
Does anyone have problem like me?
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