Nigel,
The allowed clock for Concerto is limited based on the part that is detected by the Flash Programmer. Based on my information, the F28M35H52C1 part should support a higher clock speed that the value you are currently using.
So, the Flash Programmer must be detecting your device as a different part. If you generate the DS logs (instructions: http://processors.wiki.ti.com/index.php/Troubleshooting_CCSv4#Debug_Server_Logging), I can help you look into what the Flash Programmer is detecting as the part, but it might not help with figuring out what the underlying problem is.
Thanks,
Ricky