I have had the same problem with my TT. I was lucky, that another, purely mechanical, problem came up, so now I am waiting to get an exchange unit from Palm. In the meantime I have borrowed a TT from a friend, who has upgraded, and surprisingly enough this has not given me one problem for one week now in spite of the fact, than I have installed precisely the same software on it and is syncronizing it to the same two PC's. I used BackupBuddy VFS to rebuild it after first hard resetting it, so the configuration must be the same as on my old unit (including username and preferences).
The only difference is, that my unit was US-english and the other is EFIGS.
As my experience seems to rule out any of my additional software to cause the problem, I can only conclude, that the problem rests with the Palm itself - and maybe only with the US-english version?
It would be interesting to learn, if my experience is just coincidental, or if other users have found the problem to occur only in the US-english version?