I have my ST Sim card in it and it has not rebooted in 1:43:50 and counting. Up until that time (1:43:50) I had my old T-mobile Sim card and yes I had the micro-sd in and it didn't reboot for 20+ hrs.
Here is what I did before I inserted my ST sim card.
1.) turn off phone (lol)
2.) take battery cover off
3.) take battery out
4.) remove old t-mobile sim card
5.) remove micro-sd card
6.) insert ST sim card
7.) replace battery
8.) turn on phone
9.) waited till it was fully booted and inserted micro-sd card (2gb)
10.) replace battery door
so that right there is working for me as it has not rebooted yet (1:50:00 uptime). so I am keeping my fingers crossed and hope that it doesn't reboot.
I am thinking of selling this phone to get some of my money back because I am really frustrated with it rebooting.
I have noticed now that it reboots more when i'm not at home, so I don't know if its a WIFI/APN that is screwing it up.
Earlier today I went to the gas station and while I was waiting in line (gas was cheap @ 3.35) and I wanted to check my face book and when I clicked facebook for andriod it rebooted and rebooted for like 30mins (I was heading out of town) so when I finally get to where I was going I took the battery out and removed my SIM card and its a good thing I brought along my Nokia phone.
I just did a factory reset (its like my 5th or 6th time) and if it reboots again them I'm selling it along with my other old T-Mobile phones and maybe an unopened PS3 game (Need for Speed Undercover).
The only apps I have installed are facebook for andriod and apn manager and google maps updated itself.
The only thing I can suspect is that maybe the SIM isn't making good electrical contact and that's causing the rebooting. Other than that, it's also possible your phone is just on its way out and it might just be time for something different.