Up until 2 days ago I was rooted, had VirusROM with the stock radio, and had the "stock" radio. Never had a single reboot or any problem whatsoever.
I now have the new radio and a new ROM, and still haven't had an issue.
So I have been rooted since about 2 weeks after I got the phone, had a custom ROM since a couple days after that, and have never had a random reboot or any type of problem.
I have only had maybe 2 reboots the whole time i've had the tbolt - since the day after it came out basically.
some phones (actually, lots of phones) reboot with certain radios. Some phones were rebooting with the original radio, and, notably, alot of phones started rebooting AFTER they got the first OTA from verizon.
Anyway, the reason for the reboots is likely in the radio - and that's why there are 4 froyo radios made available by the root community - The OTA froyo, the old froyo, and 2 combinations of those 2 radios.
Now there are two new radios, mr2 and mr2.5, which currently work with the Gingerbread sense roms for tbolt, but will be working with froyo mr2-ril based roms as they begin to come out (i understand that flyweight is one of these to come out so far).
basically, if you ahve reboots, one of these radios should work for you - if you want gingerbread, either mr2 or mr2.5 should work, and if you want froyo, one of the four should work pretty well for you i hope.
either way, there are radios available to try if you are rooted. if you're not rooted, and you have reboots, i would suggest returning your phone for a different handset that HOPEFULLY doesn't have this issue.