when you changed your tube did you change your tire? the reason i ask, is if there is something stuck in your tire that popped the tube, and you only changed the tube, its probably still there, popping your tube. also, what type of valve are you using? i received two presta valve tubes, both times the valve core leaked. not sure why. the bike shop where i live sells the valve cores, which fixed the problem. also, i would absolutely recommend getting self sealing tires, or the slime stuff. and "mr tuffy" tire liners. i put two tire liners in each tire, their fantastic. also, make absolutely sure that you have something in between the tube, and the rim, because the little nuts or whatnot that connect the spokes to the rim could be popping the tube if one of them is a little too sharp. i have a layer of gorilla tape, then a rubber thing that i found on another tire, thats kinda like rim tape. this might not be the best way to do it, but after replacing my rear tire, and tube, and using gorilla tape, the rim tape, and mr.tuffy's im yet to have a problem. and i did indeed take my bike offroading a few times.