Hello to all,
I have also had the same problem on a 930T, when installing v8.35 and also 8.351, and also with a 720 of a frien of mine, but I have solved it.
I found that the problem was NOT the software upgrade itself, but a strange behaviour of TomTom Home not correctly disconnecting the unit; this way the file system was corrupted, and the unit constantly booted and resetted itself. Note that Home has shown this problem only when installing v8.35 and v8.351, and not in other usages.
To solve the problem reconnect the unit to the PC (but without Home turned on, so check also in the system tray to close it completely), and first of all make a ScanDisk of the internal memory (you will see that some corrupted files will be found and corrected: you can then safely delete the dir \FOUND\ on your TomTom, which is containing the corrupted files).
Next step, uncompress MANUALLY the v8.351 software to your unit (on a Vista OS, you will find the compressed downloaded CAB software in C:\Users\OWNER'S NAME\Documents\TomTom\Home\Download\complete\program\ , and here search for a file named 8.351.go530-go730-go930.CAB-navcore-GO.CAB ) . Decompress it with something lilke WinZip, and manually copy the uncompressed files (together with its file structure) to the default dir of the internal memory of your TomTom.
Voil?, the software will be correctly installed! And you can use this procedure also for manually installing future upgrades, if Home will have strange behaviours also in the future...
NOTE: Please follow the instructions carefully, and ALWAYS make a backup of the unit before managing manually its internal memory!