The funny thing is we had it rock solid before we made the CPU timing 100% exact. So I am guessing there is some subtle timing thing going on. But I won't have time to look until we finish moving. So any investigations you are able to do in the meantime would be great. For example, make a routine that toggles the IEC lines at some known fixed frequency, and compare from real C64 to MEGA65 to look for differences, e.g., if the frequency is wrong, or there is a lot of jitter.