<VV> Carb Heat-Related Issue

[Grant Young]

After having rebuilt thousands and thousands of Rochesters, I thought I had seen everything as well as screwing up everything at least once. But I have come across an issue with some '65 secondary carbs that I hope someone else has encountered. The carbs seem to fit and seal properly. They have the requisite new throttle shafts and good run out. They do not leak air at idle. The car starts up and runs fine for the first 10-15 minutes or 30 miles or so, but when the driver attempts to use them after that they are "locked" in place and can not be opened with the gas pedal and linkage. They work fine on the engine prior to that place in time. All the linkage is installed correctly with no excessive play anywhere. It appears to me to be heat related, but I don't know how to correct it since the throttle body areas have already been re-polished in an attempt to correct it. (The plates are installed correctly so the angle matches the bores). Any ideas?
Thanks,
Grant