Ski stability is no longer a function of length.
Improvements in construction (dampening materials), etc. make short(er) skis that are at least as stable, if not moreso, than our old long sticks.
More importantly, since they are sooo much easier to get and keep on edge, that in and of itself makes them more stable.
If you are bombing down the hill and trying to keep yer skis FLAT (a la downhill or maybe Super G) then they likely will be unstable.
The key is to always be edging, be it ever so slightly, and then they feel like they are on rails.
Bottom line: new skis are a VERY big investment. While there really are no longer any full-on crappy skis being made
, you will enjoy it much more if yer skis do what you like to do well.
That super-duper mid-fat freeride ski that all the magazines loved (for out West) probably won't be as much fun on boilerplate at Stowe or Hunter.
And the best way to find that out is to demo. Most larger ski mtns have a demo center. Or demo days.
Pick the 3 or 4 models you think will be best for you and take 'em all out for a test drive.