It's ridiculous to attempt to measure intelligence based solely on how many verbal gaffes one can collect.
Consider: The average number of spoken words per day by an individual, male or female, is around 16,000. Multiplied times 365 and you've got 5,840,000 words per year. You have to figure a President, between the dozens of formal speeches given during the year and the constant interpersonal communications required by the job, probably averages twice that or more. Multiply that times 4 years for a term or 8 years for a two-termer and you're looking at close to 50,000,000-100,000,000 spoken words.
Hell, I consider myself fairly well-spoken, but I stutter and have to back up and start over multiple times each day. I have slips of the tongue. I mishear or misunderstand what another is saying and respond out of context. I'd hate to see what they could come up with if there was corp of media people recording and scrutinizing every word out of my mouth.
Yet people can collect a dozen or so words and phrases from George Bush and play them looped over and over against a laugh track and use that as some measure of his overall intellect and ability. It doesn't wash. He has no more of a problem with the English language than Obama or Biden, it's just that every slip they make isn't collected into a Greatest Hits album to be replayed over and over for the public's edification like you see done with Republican pols.