You are right, McCain was campaigning in grocery stores when Palin came along and gave him a slight bump. Crowds were nowhere near Obama's but she did okay until she started doing interviews. People began to think about McCain's age and her being first in line of succession and attitudes toward her changed in a big hurry.
The press labeled her as an idiot. Once the meme started that was it.
Granted, Palin doesnt interview well but the level of vehemence against her was unprecedented.
But ultimately, as always, it is the candidate who carries the election. Bush won with Quayle on his ticket, Nixon won with Agnew. McCain lost because of McCain.