C
There was a time when a LOT of Pronto's had this problem. The reason was that Philips released a firmware update that made this problem worse. Don't know if this was also the case with the Marantz's or not. First I'd check my firmware revision on the remote, it should be App v5.1 and Sys v 4.91. If it's not, this could be the source of the problem.
You mention the "hyperterminal fix". From my understanding this is not a fix for your problem. It's intended to repair a "dead" remote or one that's in a continuous reboot.
Common causes for a reboot are the firmware (try reflashing even if it's current) and low battery power (does it still happen after a full recharge).
Bad codes can cause reboots, but this is an uncommon situation. Does the reboot occur with specific button(s) or macros, or does it not seem to make any difference? If it seems to be certain buttons or macros, try replacing/relearning those codes. Hope this helps at least a little.
Jim L