thanks very much for that Tom,
I tried the new core but unfortunately the results were still the same, it seems like what ever difference there is between hardware and software serial, is the difference between this working and not working.
the only thing i can think is happening is that the ATtiny is picking up a serial byte right in the middle, this would then be rejected by the switch case as not a valid input and for a fraction of a second the ATtiny would stop transmitting IR, and the receiver might interpret that as a button debounce on the remote control.
it works fine if i have separate commands that drive the hold functions, and basically loop round and retransmit the IR signal about 16 times, but thats not really what i was hoping for. my original idea was going to use an ATmega382 but it seemed like such a waste for such a simple job, but the lack of hardware serial on the ATtiny85 makes it very difficult to do what i want to do.