Phone tech help?

I finally got a memory card for my Samsung Alias 2 phone, but when I copy MP3s onto it and say "scan card for new music" the phone always randomly *reboots* after it's processed 20-100 of them. The songs already processed work just fine, but further attempts immediately fail. (The 4GB microSDHC card is the same brand that Samsung sells on their site under "accessories" for this phone.) What the heck is going on?

[I've already tried: 1. Repartitioning the card into two 2GB partitions, just in case there was a size limit that I didn't know about (and that Samsung was ignoring in their own store). I went back to a single 4GB partition after that didn't help. 2. Deleting all the irritating auxiliary file cruft that my Mac saves onto the card alongside the actual music. 3. Loading files in smaller chunks. (The first 35 song chunk worked fine, but it rebooted during the next 33 song chunk.) But I've had it crash as early as song #24 or so, too: I've yet to identify a pattern.]