Nokia 6230 Freezes

I upgraded to a Nokia 6230b with Cingular just over a week ago, and
I'm having a problem with the phone periodically locking up. When it
happens it won't respond to anything--I just have to remove the
battery to reset it. Alternatively, sometimes when I go to my contact
list it just starts scrolling down by itself & again won't respond,
requiring me to remove the battery. I upgraded just over a week ago &
am already on my second 6230 (the store replaced the first one when I
took it in while in auto-scroll mode), and now the second one is
starting to act the same way. Is it possible my SIM card is bad? What
else could be causing this? I searched & couldn't find anyone else
with similar problems, but I've had two phones in a row start acting
this way in less than a week. After the hard reset it works fine for a
while, but after several hours starts acting up again. My firmware is
v 05.34.

Thanks for any help!

--
Greg