As long as you don't have slack in the chain, it should be short enough (for whatever gear you're in). Obviously, you're most likely to get slack in the small ring-small cog combination. Since the chain is jumping around when you are in the small ring, that's where you should be checking for slack. If you have slack, you can shorten the chain, as long as it remains long enough to shift into the large ring-large cog combination. (Some people will shorten it beyond this and just avoid the large-large combination, but if you forget and try to go there, it can really mess things up. I wouldn't do it.) If the chain is already as short as you can go safely, you might consider running a derailleur with a longer cage, if you don't already have one.

It's possible, I think, depending on the nature of your frame and your size and weight, that if you're really stomping on the hills, your frame might be flexing enough to cause the ghost shifting. Chain length might not be the problem.