PXA27x SD/MMC card CRC errors - only sometimes

I've been banging my head against a problem with the PXA SD/MMC driver 
for a couple of days and I thought I'd fish for any suggestions here on 
what might be going on.

I have the driver working on the Palm Treo 650 for *most* SD cards, but 
certain cards cause it to get back invalid CRC data whenever it attempts 
to read from the card.  Since it works in some cases, I guess it's just 
some quirk of the card itself that isn't handled in the driver.

I tried clocking it down below 19.5MHz (to avoid the PXA27x errata that 
indicates that some cards might be a problem at that speed), but that 
didn't seem to have any effect.

I figured I'd fish for anything here before I start building up a test 
program from scratch to figure out what might be failing.  Any ideas?

Thanks!
Matt