I've had the same issue as well... Could be multiple things.

1) Always always check the inside of the tire for protrusions and debris. You can generally (but not always) feel them. A piece of wire, a piece of glass, a thorn, whatever. Make sure you're starting clean!
2) Always mount your tires the same way -- I always put the manufacturer logo in line with the valve stem. Compare the puncture location on the tube with the same location on the tire. What passed the feel test (step 1) may show up in a visual examination. Sometimes a piece of debris embedded in the tire won't rear it's ugly (and sharp) head unless under pressure.
3) Always partially inflate the new tube by mouth to give it some shape.
4) Always mount the tire/tube back on the rim by hand if possible, being careful not to pinch the tube.
5) Check for pinches all the way around before adding any air
6) Add ~30-40 PSI and check for pinches again
7) Fully inflate, listen for leaks, etc.

Sounds obsessive but I've had your problem before.

And, as you mentioned, sometimes you do get bad tubes. I've had a batch of bad ones and it drove me absolutely nuts. I ordered another set of tubes and suddenly the blowouts stopped.

Good luck! I hope you find the root cause.