Hum ... it appears that the disk geometry that the partition table reports is ither incorrect or incompatible with linux.
I wiped out the whole partition table data stuff by writing 0 ro the first 10Mb of the disk with this:
dd if=/dev/zero of=/dev/<whatever it is on your sustem> bs=1024 count=10240
Then I used fdisk to make a new empty dos layout and then created the linux partition and swap ... since then I've been able to reboot without getting file system corruption.
So I now have my slack 12.1 on the 8Gb SDHC.