I'm no expert but I've heard a good number of no start problems are the ignition switch assembly going bad... The switch itself and/or the passlock security goes flaky... More often than not replacing the ignition switch solves the problem... But wait for some more "experts" to respond to you... Also check out the website
www.naioa.com and post your problem there...
naioa=North American Impala Owners Association
Good luck!