Transient errors on SD card

I just received my eSATA Sheevaplug with a pre-installed Transcend 8GB SDHC card (Ultimate class 10). I started playing with my new toy, booted Ubuntu from the SD card, upgraded, installed lighttpd, php5, mysql-server, phpmyadmin, phpsysinfo. I could not install vim because apt-get failed on a "Exec format error" but I didn't really need it so I left the issue for later. And then, I just couldn't continue working because the SD card suddenly became read-only ('read-only file system' error when trying to modify things). 'df' was still showing rw partitions but they were ro. An 'ls -algs /' issued 'Input/output error' on 3 directories (among which /opt) and finally printed some lines like:
0 ?????????? ? ? ... ? opt
and some others that looked normal:
0 drwxr-xr-x 55 root 8264 Jan 17 08:02 etc
After a cold reboot things seemed to be OK but after a few minutes it started again. Same after a reset. The plug operates fine for minutes or hours and then the same problem occurs again. Yesterday I booted on the internal flash. Things look fine up to now. I inserted the SD card, mounted its two partitions:
$ mkdir /mnt/sd1 /mnt/sd2
$ mount /dev/mmcblk0p1 /mnt/sd1
$ mount /dev/mmcblk0p2 /mnt/sd2
A 'badblocks' on both didn't reveal any bad block. I had a look at /mnt/sd2/var/log/syslog but nothing special: the kernel probably could not write error messages at the time it would have been useful. Any idea on where it comes from and what to do next? Thanks.