According to another forum (Drippler) the fix is being worked on ny HTC and Verizon.
HTC, Verizon working on ThunderBolt reboots by Phil Nickinson When it comes to the HTC ThunderBolt, probably our biggest gripe is the spate of reboots. Look down, and, yep, there it goes again. It's one thing if it's a hacked phone with nightly builds --and another if it's a stock consumer release. But the good news is that some whiny blogger whined about it on Twitter, and Verizon responded with what you see above. Not that we had any doubt that HTC and Verizon are working to squash a bug or three, but it is nice to see it publicly. Source: Twitter; More: ThunderBolt forums (1, 2, 3, 4)
HOUSTON THUNDERBOLT