wich update and how did you install it?
if you mean the latest firmware 1.13.607.7,and you installed it manually thru hboot,you might be having issues with that radio.it has been know to cause that prollem for some.
if you mean the 2.3 leak,that is a pre-release version that will have issues and problems. some have trouble with the stock kernel. some have trouble with imoseyons kernel,wich fixes alot of other things.
so...
-you might need a different radio
-you might need a new rom
-you might need a new kernel
-you might need all the above
give us more info on what you were running,what update you applied,and how you applied it,and what youre running now,and we can help you better.
edit: to answer your question: no,it should not reststart onits own. i personally will not run any software(rom,radio,or kernel) combo that gives me multiple random reboots. no matter how cool it is.