Aside from the crashing, is there some problem with or trick to actually using normal maps? I bought a stock human model that has them, and using the skylight or an area light seems to hurt the effect, the underlying mesh structure gets highlighted.
I haven't seen the effect you're describing. Can you send a screen shot or portion of the file that reproduces this? I have used normal maps in both the normal and bump channels. The additional area light would be the first element I'd remove from your description. I imagine this light type might cause strange effects with some normal maps given it's indirect nature.