GG continues his advice to use shorter URL parameters.
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Hmm. My advice is that since the identifier is random, don't make it 18 letters. Looks like you're using uppercase and lowercase, so maybe it's base 64 encoded. 64 possibilities in 18 slots = 64 * 64 * 64 ... * 64 a total of 18 times, = 64^18 = 324518553658426726783156020576256 possible articles (check my math). I'm guessing that you don't really have that many pages--maybe it's more like a million? In that case, just use 4-6 digits or so and you'll still have plenty.
18 random letters will look an awful lot like a session ID to a search engine spider. :)