but they've been different brands from different places

the weirdest thing is that it's only the back tire. the front has never popped and deflates at a normal rate.

you have gnomes

that was my first thought, but figured i'd put it out there to see if someone could up with a different idea

are you using levers to install the tire? if you are, stop. you should be able to do it without a lever (on most tires, not all).

FWIW if I inflate my 700/23 front to 120 psi it will go back donw to about 100 in the course of a week or so. I usually run it at 100 and it stays at 100 for a really long time. I probably reinflate once a month. if I'm going on a long ride or working out alot I will keep em at 110 and inflate em once a week. not telling you what to do just trying to give you an point of reference.

when I was first getting into bikes (after a long bikeless hiatus) I ruined a bunch of tubes trying to install them with levers. then a good friend showed me how it is really done.

when you say the tubes "popped" does that mean they actually went bang or just that they no longer hold air?

no levers, and they were punctured. punctured from unrelated instances.

Whatever one does for fun or for work, one is better off knowing the the ins and outs of the job, sport, task, object. If you tires are leaking constantly, you can bet that its YOUR falt, for not knowing what you are doing.
Like the one guy said, if your stem ripps away from the tube look at its seat, maybe it has a sharp edge, figure out how to smooth it. If your new tubes keep leaking, find what is puncturing them, probably something still in the tire. The nut on the presta valves sould be hand tite, not dicarded as is true for the caps. If you valve is bent, replace it. If your LBS can't fix it the first time, find a new BS. The only probs I've had with flats have been Goat-heads or snake bites, and once a valve that got junk in it (shrader), so I cleaned it out. If you realy like biking but your bike came from from a dime store, maybe look to a better bike. there are a few good bike review sites on the net, do your homework, read up on the right equiptment. OR, keep being a victim, your choise.
In other word people, take responsability for your actions and things and you'll no longer have such problems.

You know this thread is over a year old, right?