Frank, after following the textbook guidance regarding priming sugar for my first couple of years brewing, getting way too many gushers, I decided to cut back on priming sugar a little bit. For example, try cutting back to like 90% of what the rules of thumb would dictate. You'll still get plenty of carbonation, no worries about that, but without as many gushers. If 90% is still too much, then cut back to 80% -- incidentally, that's about where my beers are carbonated now. I still get a gusher on rare occasion for whatever reason, but it is extremely rare that I get a beer that is nearly flat. Most beers are perfectly carbonated. So that is my advice. Not sure exactly why it happens, and there are a lot of variables involved. However I do think the rules of thumb for priming sugar set the amount too high......
Just a possibility, but recommendations to overprime, at least in a few cases, might even be on purpose in order to guarantee success for beginning homebrewers who are always too eager to pop their first 6-pack, and by priming a little bit too much on the high side, they are guaranteed some carbonation when their impatience wears thin. Once they learn that the beers become gushers with age, they can choose whether they want to cut down on priming suger. For lazy brewers like me, I can wait a month before opening a bottle of beer. Sometimes. And by then, carbonation is done. Or often I will set a couple of bottles in a warm spot in my house (75 F) so that they carbonate within about one week, and leave the rest in my basement to carbonate in a cool environment (60 F). By the time the first couple are gone and I am craving some more, the ones in the basement are usually ready to roll.