SDHC cards have warranty, use it, if you're having trouble within 2 months of life. Also, use Kingston instead, they're the best, and offer unlimited warranty.
They do have a ~100.000x read/write life cycle, but i never saw one ending.. Even my 7/8 years old 1GB MMC card which was used a few years in a N-Gage (lots of read/write), and then a few more with BT3, still works. I'd say what's happening to you is very odd..
Just an advice, if you're buying 8Gb SD cards, note that with the price of 2, you can buy an ext 500Gb USB HDD.. I'm basing myself on the local prices, but it shouldn't be much different wherever you're from.