Chester,

I had a similar issue when I upgraded to WM6. It was not related to
the memory card, but the older SIM card that I was using (pre 2002 is
what I was told). It worked fine with WM5, but after the upgrade began
doing this soft reboot over and over until I removed the battery.

After I picked up a new SIM card from the T-Mobile store everything
began working perfectly. Thankfully they did not charge me.

As a test, try turning your phone on without the SIM card. If it
starts fine, but alerts you that you don't have a SIM card in the
phone this is probably the problem.

Good luck!