I experience the same deal: image of the woman changes slightly while personality traits remain the same.
I have had no trouble getting dates since after high school, when I decided I needed to 'learn' how to interact with women once and for all. I read a ton of books on the subject; some good and some lame.
Eventually I 'got it' pretty much but I still do what you're talking about in hopes of attracting a woman who isn't just attractive or intelligent, but is really like long-term material.
So I think it's totally normal to have the image of her fluctuate as your visualizations progress: your brain may be telling you 'her exact physical make-up isn't all that important, as long as she falls into this range, but these personality traits are a must'
Make sense? Also, here's a possible reframe. You said you don't want a woman who is frigid in bed, yet you had issues with a girl who seemed to be apparently a little *too good* in bed.
Chances are, many women in that boat tend to have somewhat less-than-saintly pasts. If a girl appears inexperienced -- she may not be 'frigid' toward you -- maybe she just doesn't know what you like. You could try teaching her some new things...
I would find this infinitely preferable to a woman who's been around the block one too many times.
Just my thoughts.
Dave