IOS help, ATM URB and IPv6

Hi NANOGers, I am looking for some help with a IOS problem. Got a 7206 VXR NPE-300 with a DS3 ATM interface and a couple of PA-FE's its running c7200-js-mz.122-10a.bin This version of IOS solved a critical ATM UBR cell rate problem, but at the cost of IPv6. I've tried a stack of other versions all purporting to support UBR and IPv6. The v6 stuff works, but UBR breaks by sending cells faster than the ATM provider will accept and thus drops AAL5 frames. TAC hasn't been able to get us a good version. so I'm turning to the wisdom of NANOG for any ideas on what version to try. mucho thanks john brown