I've noticed the difference, too. Presidente used to have much more of a "kick" then it does now. A LOT of my friends have noticed the same. Right on the bottle it says 5% alcohol by volume. That's the same as most American beers.
Back around 2000 a 12oz. cost around $.60 and a 22oz was under a Dollar US. Now a 12oz is about $1.50 and a 22oz almost $2.00 US. I know taxes have gone up on alcohol - so, maybe they reduced the alcohol content to try a keep prices from going any higher and save on costs(?). I can't prove it, but years ago I could swear Presidente was 6.0% - 6.5% alcohol by volume.
Even if I'm wrong on that, as quite a few other posters have said, the green bottle and long storage times in overlywarm conditions will negatively affect the taste of the beer.