Possible reasons for your problem:
a)new, different provider with unstable network
b)different usage, e.g. ActiveSync renning in the background
b)the new phone
My phone sometimes crashed when using data transfer and ActiveSync in parallel, worSt if using AS push services.
If I install support for chines/korean or japan languages it'll crash at evvery data transfer.
Maybe it's similar for you, and you could try a firmware update, if available.
My phone is more or less noname, wonder why you have such problems with a htc. Maybe it's broken.