If your problem “sometimes”, or “randomly” happens, there’s not much I can do to track it down: http://www.kongregate.com/forums/9794-mutilate-a-doll-2/topics/725152-bug-reports#posts-11120455
Negative gravity is up, because that's how physics work. You can also click on a slider's title to be able to input numbers, and click again to apply them.
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There is no limit, other than performance I guess. Which is not something I can really magically lift since it's both engine and platform-dependant.