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Try posting this on the tramsmissions page (scroll down a bit from here), there are a couple of good transmission experts visit there.
My thought was a dirty filter but you have ruled that out. Another possibility is a badly fitting filter however I assume it was doing it before the new filter. It could also be a stuck drain back valve allowing fluid to flow back from the converter when the truck has been sitting. You could do a search of the transmission page for more info on that.
Hope this helps.
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